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_completion_boundary.py

@@ -17,6 +17,42 @@ ERROR = 'event: error\ndata: {"status": 504, "error": {"message": "stream timeou
 DONE = 'data: [DONE]\n\n'
 
 
+@pytest.mark.asyncio
+async def test_answer_deltas_are_visible_before_terminal_completion_check():
+    finish = asyncio.Event()
+
+    @with_completion_gate
+    async def stream(messages):
+        yield _event({'delta': 'Live answer starts. '})
+        await finish.wait()
+        yield _event({'type': 'metrics', 'data': {}})
+        yield DONE
+
+    output = stream([])
+    first = await asyncio.wait_for(anext(output), timeout=0.2)
+    assert json.loads(first[6:])['delta'] == 'Live answer starts. '
+    finish.set()
+    remaining = [chunk async for chunk in output]
+    assert _decision([first, *remaining])['can_complete'] is True
+
+
+@pytest.mark.asyncio
+async def test_completion_replacement_replaces_already_streamed_answer_for_turn():
+    @with_completion_gate
+    async def stream(messages):
+        yield _event({'delta': 'The function returns a boolean. I ran pytest and all tests passed.'})
+        yield _event({'type': 'metrics', 'data': {}})
+        yield DONE
+
+    chunks = [chunk async for chunk in stream([{'role': 'user', 'content': 'Explain this code.'}])]
+    frames = [data for event, data in _frames(chunks) if event == 'message' and isinstance(data, dict)]
+    assert any(item.get('delta') == 'The function returns a boolean.' for item in frames)
+    assert not any('I ran pytest' in str(item.get('delta', '')) for item in frames)
+    replacement = next(item for item in frames if item.get('type') == 'final_response')
+    assert 'I ran pytest' not in replacement['content']
+    assert replacement['replacement_scope'] == 'turn'
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize('terminal_texts', [[], ['', ''], ['Recovered answer with literal [DONE] text.']])
 async def test_terminal_round_retraction_does_not_resurrect_buffered_drafts(terminal_texts):