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_readme_ascii_fenced.py

@@ -1,7 +1,11 @@
 """Regression guard for the README title presentation.
 
-The original ASCII-art banner needed a code fence to preserve whitespace.
-Keep the wordmark title identity and the historical ASCII-fencing guard.
+Originally (#1390) the README opened with an ASCII-art banner that had to live
+inside a ``` code fence, otherwise GitHub's markdown collapsed its leading
+whitespace and box-drawing rules and rendered it misaligned. The README refresh
+(#4306) dropped that banner in favour of a centered wordmark image, so the guard
+now pins the wordmark identity instead, while still catching the original failure
+mode if an un-fenced ASCII banner is ever reintroduced.
 """
 from pathlib import Path
 
@@ -19,6 +23,8 @@ def _fenced_segments(text: str):
 
 
 def test_readme_opens_with_wordmark_title():
+    # The README must still open with a recognizable Odysseus title: now the
+    # centered wordmark image rather than an H1 / ASCII banner.
     head = "\n".join(README.read_text(encoding="utf-8").splitlines()[:15])
     assert 'alt="Odysseus"' in head, "README must open with the Odysseus wordmark image"