fix(browser): stop treating a missing cmdline as proof the daemon exited

_terminate_owned_daemon() read /proc/<pid>/cmdline and, on FileNotFoundError,
unlinked the pid file on the stated assumption that "the daemon may have
exited". Off Linux that file is always missing, so the branch always fired:
the pid file of a live daemon was deleted and the daemon itself never killed.
_owned_daemon_exists() swallowed the same error and therefore always returned
False, which is precisely the state its own docstring warns about, since a
close against an unrecognised session can bootstrap a fresh daemon and wait on
its browser indefinitely.

Demonstrated on macOS before the change: a pid file holding a live pid is
removed by _terminate_owned_daemon() and _owned_daemon_exists() reports False.
After it, the file survives and the daemon is reported present.

_process_command_line() now returns None for "this host cannot tell" and
_process_is_alive() answers the separate question of whether the pid exists.
Without procfs we decline to kill a process we cannot confirm is ours, and we
only forget a pid file once the pid is genuinely gone. Linux behaviour is
unchanged: the command-line identity check still gates both paths.

tests/test_private_browser_tool.py

@@ -1845,3 +1845,81 @@ def test_terminate_owned_chrome_kills_only_this_runtimes_profile(
     PrivateBrowserTool._terminate_owned_chrome({"TMPDIR": str(tmpdir)})
 
     assert killed == [101]
+
+
+def _pid_file_for(tmp_path, monkeypatch, namespace, session, pid):
+    """Write a pid file where the daemon helpers will look for it."""
+    monkeypatch.setenv("XDG_RUNTIME_DIR", str(tmp_path))
+    monkeypatch.setenv("ODYSSEUS_BROWSER_NAMESPACE", namespace)
+    candidates = web_tools._browser_pid_file_candidates(tmp_path, namespace, session)
+    target = candidates[0]
+    target.parent.mkdir(parents=True, exist_ok=True)
+    target.write_text(str(pid))
+    return target
+
+
+def test_live_daemon_pid_file_survives_a_host_without_procfs(
+    monkeypatch, tmp_path
+) -> None:
+    """Off Linux a missing cmdline is not evidence the daemon exited."""
+
+    monkeypatch.setattr(web_tools, "_PROC_ROOT", tmp_path / "no-procfs")
+    monkeypatch.setattr(web_tools, "_process_is_alive", lambda pid: True)
+    killed: list[int] = []
+    monkeypatch.setattr(web_tools.os, "kill", lambda pid, sig: killed.append(pid))
+    pid_file = _pid_file_for(tmp_path, monkeypatch, "clawmm-test", "session-1", 4321)
+
+    PrivateBrowserTool._terminate_owned_daemon({}, "session-1")
+
+    assert pid_file.exists(), "a live daemon's pid file must not be removed"
+    assert killed == [], "an unverified process must not be killed"
+
+
+def test_dead_daemon_pid_file_is_removed_without_procfs(monkeypatch, tmp_path) -> None:
+    """A pid that no longer exists is the one case that justifies forgetting it."""
+
+    monkeypatch.setattr(web_tools, "_PROC_ROOT", tmp_path / "no-procfs")
+    monkeypatch.setattr(web_tools, "_process_is_alive", lambda pid: False)
+    pid_file = _pid_file_for(tmp_path, monkeypatch, "clawmm-test", "session-2", 4322)
+
+    PrivateBrowserTool._terminate_owned_daemon({}, "session-2")
+
+    assert not pid_file.exists()
+
+
+def test_owned_daemon_is_detected_from_a_live_pid_without_procfs(
+    monkeypatch, tmp_path
+) -> None:
+    """Answering "no daemon" here is what lets close bootstrap a fresh one."""
+
+    monkeypatch.setattr(web_tools, "_PROC_ROOT", tmp_path / "no-procfs")
+    monkeypatch.setattr(web_tools, "_process_is_alive", lambda pid: True)
+    _pid_file_for(tmp_path, monkeypatch, "clawmm-test", "session-3", 4323)
+
+    assert PrivateBrowserTool._owned_daemon_exists({}, "session-3") is True
+
+
+def test_owned_daemon_absent_when_the_pid_is_gone(monkeypatch, tmp_path) -> None:
+    monkeypatch.setattr(web_tools, "_PROC_ROOT", tmp_path / "no-procfs")
+    monkeypatch.setattr(web_tools, "_process_is_alive", lambda pid: False)
+    _pid_file_for(tmp_path, monkeypatch, "clawmm-test", "session-4", 4324)
+
+    assert PrivateBrowserTool._owned_daemon_exists({}, "session-4") is False
+
+
+def test_procfs_host_still_matches_on_the_command_line(monkeypatch, tmp_path) -> None:
+    """With procfs present the identity check stays exact, not pid-liveness."""
+
+    proc = tmp_path / "proc"
+    (proc / "5555").mkdir(parents=True)
+    (proc / "5555" / "cmdline").write_bytes(b"node\0agent-browser\0--serve")
+    (proc / "6666").mkdir(parents=True)
+    (proc / "6666" / "cmdline").write_bytes(b"some\0other\0process")
+    monkeypatch.setattr(web_tools, "_PROC_ROOT", proc)
+    monkeypatch.setattr(web_tools, "_process_is_alive", lambda pid: True)
+
+    _pid_file_for(tmp_path, monkeypatch, "clawmm-test", "session-5", 5555)
+    assert PrivateBrowserTool._owned_daemon_exists({}, "session-5") is True
+
+    _pid_file_for(tmp_path, monkeypatch, "clawmm-test", "session-6", 6666)
+    assert PrivateBrowserTool._owned_daemon_exists({}, "session-6") is False