feat(harness): authority, turn contract, tools, clean_agent_preview

Chat/agent harness work: request-authority model, turn contract, tool
policy/parsing/execution, clean_agent_preview streaming/thinking, ask_user
persistence, session probes, and scheduler.

Source: vendor/photocraft (ad1cff44) harness files, 3-way merged onto
upstream/dev (a8c147b2). Harness core files were independently developed
in both lineages; resolution policy documented in pr_work/C_conflicts.md
(vendor kept where ahead, upstream kept where vendor is behind; full delta
preserved in pr_work/C_upstream_delta.patch).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_pr6020_rebase_regressions.py

@@ -240,7 +240,8 @@ def test_non_qwen_fallback_keeps_requested_temperature(monkeypatch):
 
 
 def test_qwen_notes_fallback_reenables_personal_managers(monkeypatch):
-    """Model/route fallback cannot bypass caller-disabled tools; denials persist."""
+    """The answering candidate's notes mode must unblock the managers for
+    execution, not just enable them in its own route schemas."""
 
     _install_route_probe(monkeypatch)
     stream_round = 0
@@ -298,9 +299,8 @@ def test_qwen_notes_fallback_reenables_personal_managers(monkeypatch):
         )
     )
 
-    # Caller hard denials are absolute: route fallback cannot re-enable disabled tools.
-    assert {"manage_notes", "manage_calendar", "manage_tasks"}.issubset(
-        seen_exec["disabled_tools"]
+    assert seen_exec["disabled_tools"].isdisjoint(
+        {"manage_notes", "manage_calendar", "manage_tasks"}
     )