Harden update link git fixture

CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Fixed
 
+- Update-link regression tests now pin the throwaway bare repository HEAD to `master`, avoiding failures on developer machines whose Git default branch is not `master`.
+
 - **PR #2234** by @Jordan-SkyLF (refines #2207) — Three update-banner improvements: (1) Update summaries no longer repeat the same bullet under both "What you'll notice" and "Worth knowing" — visible notice items keep priority, and the secondary section is omitted when there is no distinct detail to show. (2) Update summaries now cap large commit lists (24 + probe item) before sending them to the summarizer and disclose when the summary uses only the latest commit subjects, while keeping the full comparison link available — bounds summarizer cost on large update ranges while remaining honest about coverage. (3) Update banners now wrap generated-summary links and long update text on narrow/mobile screens inside the banner instead of pushing controls off-screen. 108-line regression coverage for short-target dedup, repeated Agent-summary bullets, large-range capped input, and responsive wrapping.
 
 - **PR #2236** by @jasonjcwu — Silent failure detection in `api/streaming.py` now scans only NEW messages, not the full conversation history. Pre-fix, the `_assistant_added` check at `_run_agent_streaming` scanned all messages in `result["messages"]` (including pre-turn history); if any prior turn contained an assistant response, `_assistant_added` was `True` and the apperror SSE event was silently skipped, leaving the user staring at a blank response after a provider 401/429/rate-limit error. Fix extracts a `_has_new_assistant_reply(all_messages, prev_count)` helper that only inspects messages beyond the pre-turn history offset (`_previous_context_messages`); applied to both the main detection path and the self-heal/retry `_heal_ok` check. 15-test regression suite covering empty/short/long-history scenarios, the heal path, and the `len < prev_count` edge-case fallback. Also includes a small alignment fix to `test_issue1857_usage_overwrite.py` so the FakeAgent message shape matches what the real agent produces.

tests/test_issue1579_whats_new_link_404.py

@@ -38,6 +38,10 @@ def _make_throwaway_repo(tmp_path, *, local_only_commits=0, upstream_advanced=0)
     """
     upstream = tmp_path / 'upstream.git'
     subprocess.run(['git', 'init', '--quiet', '--bare', str(upstream)], check=True)
+    subprocess.run(
+        ['git', '--git-dir', str(upstream), 'symbolic-ref', 'HEAD', 'refs/heads/master'],
+        check=True,
+    )
 
     seed = tmp_path / 'seed'
     subprocess.run(['git', 'init', '--quiet', '--initial-branch=master', str(seed)], check=True)