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_llm_core_thinking_models.py

@@ -7,6 +7,7 @@ os.environ.setdefault("DATABASE_URL", "sqlite:///:memory:")
 import pytest
 
 from src.llm_core import _supports_thinking
+from src.model_profiles import supports_user_thinking_toggle
 
 
 @pytest.mark.parametrize(
@@ -25,3 +26,16 @@ def test_deepseek_v4_models_support_thinking(model):
 @pytest.mark.parametrize("model", ["deepseek-v3", "deepseek-chat"])
 def test_other_deepseek_models_are_not_promoted_to_thinking(model):
     assert _supports_thinking(model) is False
+
+
+@pytest.mark.parametrize(
+    "model",
+    [
+        "deepseek-v4",
+        "deepseek-v4-flash",
+        "DeepSeek-V4-Flash",
+        "deepseek/deepseek-v4-flash",
+    ],
+)
+def test_deepseek_v4_models_expose_the_user_thinking_toggle(model):
+    assert supports_user_thinking_toggle(model) is True