fix(runtime): resolve Wave 3-S audit findings (P2-1, P2-2, P2-3)

- P2-1: reject non-process PID values (None, 0, negative integers) in pid_alive
  without invoking underlying process probe.
- P2-2: truthfully represent production external bridge executions as uncontained,
  external, non-authoritative grants carrying sanitized endpoint metadata.
- P2-3: reject writable_extra overlay bindings over protected system roots and
  their descendants while preserving legitimate scratch destinations.

tests/test_containment_contract.py

@@ -505,3 +505,40 @@ async def test_an_external_bridge_grant_claims_nothing_and_cannot_be_run_locally
     with pytest.raises(ValueError, match="does not own"):
         await containment.run(grant, "echo hello")
     assert no_spawn == []
+
+
+@pytest.mark.parametrize("protected_dest", [
+    "/etc",
+    "/etc/ssl",
+    "/usr",
+    "/usr/local",
+    "/bin",
+    "/bin/sh",
+    "/sbin",
+    "/lib",
+    "/lib64",
+    "/proc",
+    "/proc/sys",
+    "/dev",
+    "/dev/shm",
+    "/sys",
+    "/root",
+    "/root/.ssh",
+    "/home",
+    "/workspace",
+    "/workspace/sub",
+])
+def test_writable_extra_rejects_protected_system_roots_and_descendants(monkeypatch, workspace, protected_dest):
+    install(monkeypatch, mechanism("fake", 10, containment.DIMENSIONS))
+    spec = spec_for(workspace, writable_extra=(protected_dest,))
+    with pytest.raises(ValueError, match="reserved path"):
+        containment.acquire(spec, owner="session-1")
+
+
+def test_writable_extra_accepts_legitimate_scratch_destinations(monkeypatch, workspace):
+    install(monkeypatch, mechanism("fake", 10, containment.DIMENSIONS))
+    spec = spec_for(workspace, writable_extra=("/var/scratch", "/tmp/custom_scratch", "/home/testuser/scratch"))
+    grant = containment.acquire(spec, owner="session-1")
+    assert "/var/scratch" in grant.spec.writable_extra
+    assert "/tmp/custom_scratch" in grant.spec.writable_extra
+    assert "/home/testuser/scratch" in grant.spec.writable_extra

tests/test_containment_process_tree.py

@@ -376,3 +376,55 @@ def test_an_unenforceable_required_limit_refuses_instead_of_crashing_the_spawn(w
     with pytest.raises(containment.ContainmentUnavailable) as caught:
         containment.acquire(spec, owner="session-9")
     assert caught.value.missing == frozenset({containment.MEMORY})
+
+
+def test_pid_alive_rejects_non_process_pids(monkeypatch):
+    calls = []
+    real_kill = os.kill
+
+    def fake_kill(pid, sig):
+        calls.append((pid, sig))
+        return real_kill(pid, sig)
+
+    monkeypatch.setattr(os, "kill", fake_kill)
+
+    # Required: None, 0, and any negative integers must return False
+    assert pid_alive(None) is False
+    assert pid_alive(0) is False
+    assert pid_alive(-1) is False
+    assert pid_alive(-42) is False
+    assert pid_alive(-9999) is False
+    assert pid_alive("not_a_pid") is False
+
+    # The underlying process probe (os.kill) must NEVER be invoked for pid <= 0
+    assert calls == []
+
+    # Normal positive-PID behavior remains covered
+    my_pid = os.getpid()
+    assert pid_alive(my_pid) is True
+    assert (my_pid, 0) in calls
+
+
+def test_pid_alive_retains_conservative_liveness_on_eperm(monkeypatch):
+    def fake_kill(pid, sig):
+        raise PermissionError(1, "Operation not permitted")
+
+    monkeypatch.setattr(os, "kill", fake_kill)
+
+    # Positive PID with EPERM is conservatively considered alive
+    assert pid_alive(1) is True
+    assert pid_alive(99999) is True
+
+    # But non-process values still immediately return False without calling probe
+    assert pid_alive(None) is False
+    assert pid_alive(0) is False
+    assert pid_alive(-1) is False
+    assert pid_alive(-100) is False
+
+
+def test_pid_alive_reports_false_on_process_lookup_error(monkeypatch):
+    def fake_kill(pid, sig):
+        raise ProcessLookupError(3, "No such process")
+
+    monkeypatch.setattr(os, "kill", fake_kill)
+    assert pid_alive(99999999) is False

tests/test_production_external_bridge.py

@@ -0,0 +1,197 @@
+"""Tests verifying truthful representation of production external bridge execution.
+
+P2-2 invariant: external bridge execution != local containment.
+"""
+import asyncio
+from types import SimpleNamespace
+from unittest.mock import patch
+
+import pytest
+
+from src import containment
+from src.agent_tools import subprocess_tools
+from src import tool_execution as _te
+
+
+class _FakeResponse:
+    def __init__(self, data, status_code=200):
+        self._data = data
+        self.status_code = status_code
+        self.text = "error detail" if status_code >= 400 else ""
+
+    def json(self):
+        return self._data
+
+
+class _FakeAsyncClient:
+    def __init__(self, *args, **kwargs):
+        pass
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, *exc):
+        return None
+
+    async def post(self, url, *args, **kwargs):
+        return _FakeResponse({"stdout": "remote stdout", "stderr": "", "exit_code": 0})
+
+
+@pytest.fixture(autouse=True)
+def _isolated_store(tmp_path, monkeypatch):
+    store = tmp_path / "containment_grants.json"
+    monkeypatch.setattr(containment, "_store_path", lambda: store)
+    return store
+
+
+@pytest.mark.asyncio
+async def test_host_shell_creates_uncontained_external_record(monkeypatch):
+    """1. Production bridge execution creates an external/uncontained record.
+    2. It cannot be interpreted as contained."""
+    monkeypatch.setattr(subprocess_tools.httpx, "AsyncClient", _FakeAsyncClient)
+
+    tool = subprocess_tools.HostShellTool()
+    ctx = {
+        "client_runtime_context": {
+            "host_shell_bridge": {
+                "url": "http://127.0.0.1:17654/run",
+                "token": "secret-bridge-token",
+            }
+        },
+        "session_id": "test-session-host-shell",
+    }
+    result = await tool.execute('{"command": "echo host"}', ctx)
+
+    assert result["exit_code"] == 0
+    assert result["output"] == "remote stdout"
+    assert "containment" in result
+    c = result["containment"]
+
+    # Invariant: external bridge execution != local containment
+    assert c["external"] is True
+    assert c["contained"] is False
+    assert c["mechanism"] == "external_bridge"
+    assert c["enforced"] == []
+    assert c["executed"] is True
+
+    # Server-owned metadata identifies endpoint without secrets
+    assert c.get("endpoint") == "http://127.0.0.1:17654/run"
+    assert "secret-bridge-token" not in str(c)
+
+
+@pytest.mark.asyncio
+async def test_host_shell_failure_does_not_become_containment_or_effect_evidence(monkeypatch):
+    """5. Bridge failure does not become successful containment/effect evidence."""
+    class _FailingClient(_FakeAsyncClient):
+        async def post(self, url, *args, **kwargs):
+            return _FakeResponse({"error": "bridge exploded"}, status_code=500)
+
+    monkeypatch.setattr(subprocess_tools.httpx, "AsyncClient", _FailingClient)
+
+    tool = subprocess_tools.HostShellTool()
+    ctx = {
+        "client_runtime_context": {
+            "host_shell_bridge": {
+                "url": "http://127.0.0.1:17654/run",
+                "token": "secret-bridge-token",
+            }
+        },
+        "session_id": "test-session-host-shell-fail",
+    }
+    result = await tool.execute('{"command": "echo fail"}', ctx)
+
+    assert result["exit_code"] == 1
+    assert "bridge returned HTTP 500" in result["error"]
+    assert "containment" in result
+    c = result["containment"]
+
+    assert c["external"] is True
+    assert c["contained"] is False
+    assert c["enforced"] == []
+    # Failure means not executed
+    assert c["executed"] is False
+
+
+@pytest.mark.asyncio
+async def test_routed_bash_and_python_via_bridge_creates_uncontained_external_record():
+    """Prove _route_tool_via_bridge generates truthful external records for bash & python."""
+    bridge_ctx = {
+        "surface": "odysseus-tui",
+        "host_shell_bridge": {
+            "url": "http://127.0.0.1:17654/run",
+            "token": "bridge-token",
+        },
+    }
+
+    async def fake_bridge_post(bridge, path, payload, **kwargs):
+        return {"stdout": "bridge out", "stderr": "", "exit_code": 0}
+
+    from tests.runtime_evidence_helpers import server_authorized_executor
+
+    with patch.object(_te, "_bridge_post", fake_bridge_post), \
+         patch.object(_te, "_owner_is_admin", lambda owner: True):
+        # Routed bash
+        desc, result = await server_authorized_executor(_te.execute_tool_block)(
+            SimpleNamespace(tool_type="bash", content="ls -la"),
+            session_id="session-routed-bash",
+            client_runtime_context=bridge_ctx,
+            security_context=_te.NO_TOOL_SECURITY_CONTEXT,
+        )
+        assert result["exit_code"] == 0
+        assert "containment" in result
+        cb = result["containment"]
+        assert cb["external"] is True
+        assert cb["contained"] is False
+        assert cb["mechanism"] == "external_bridge"
+        assert cb["enforced"] == []
+        assert cb["executed"] is True
+        assert cb.get("endpoint") == "http://127.0.0.1:17654/run"
+
+        # Routed python
+        desc_py, result_py = await server_authorized_executor(_te.execute_tool_block)(
+            SimpleNamespace(tool_type="python", content="print('hi')"),
+            session_id="session-routed-py",
+            client_runtime_context=bridge_ctx,
+            security_context=_te.NO_TOOL_SECURITY_CONTEXT,
+        )
+        assert result_py["exit_code"] == 0
+        assert "containment" in result_py
+        cp = result_py["containment"]
+        assert cp["external"] is True
+        assert cp["contained"] is False
+        assert cp["mechanism"] == "external_bridge"
+        assert cp["enforced"] == []
+        assert cp["executed"] is True
+
+
+@pytest.mark.asyncio
+async def test_external_record_does_not_grant_authority(tmp_path):
+    """3. The record does not grant execution authority."""
+    spec = containment.agent_spec(str(tmp_path), {}, 5)
+    grant = containment.declare_external_bridge(
+        spec, owner="auth-test-session", endpoint="http://127.0.0.1:17654/run"
+    )
+
+    assert grant.external is True
+    assert grant.contained is False
+
+    # Attempting to use this grant to run local command must be rejected
+    with pytest.raises(ValueError, match="backend does not own"):
+        await containment.run(grant, "id")
+
+
+@pytest.mark.asyncio
+async def test_native_local_bash_python_behavior_unchanged(tmp_path, monkeypatch):
+    """4. Native local Bash/Python behavior is unchanged."""
+    tool_bash = subprocess_tools.BashTool()
+    ctx = {
+        "session_id": "native-session",
+    }
+    result = await tool_bash.execute("echo 'native run'", ctx)
+    assert result["exit_code"] == 0
+    assert "native run" in result["output"]
+    assert "containment" in result
+    c = result["containment"]
+    assert c["external"] is False
+    assert c["mechanism"] in ("bubblewrap", "process_group")
+    assert c["executed"] is True