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_foreground_model_routing.py

@@ -148,6 +148,7 @@ def _chat_stream_endpoint(
     endpoint_url="https://selected.example/v1",
     session_model="selected-model",
     session_history=(),
+    attachment_meta=(),
 ):
     def add_message(message):
         captured.setdefault("added_messages", []).append(message)
@@ -172,7 +173,7 @@ def _chat_stream_endpoint(
             {"role": "assistant", "content": "old answer"},
             {"role": "user", "content": "hello"},
         ],
-        preprocessed=SimpleNamespace(attachment_meta=[]),
+        preprocessed=SimpleNamespace(attachment_meta=list(attachment_meta)),
         auto_opened_docs=[],
         rag_sources=[],
         web_sources=[],