PR #29211 review findings:
1. test_retry_replacement: pin DEFAULT_DB_PATH so SessionDB() doesn't write
to the real ~/.hermes/state.db. Same fix as the other DB-only fixtures.
2. yuanbao recall branch A1 (message_id exact match) was structurally dead
once load_transcript() became DB-only — state.db never preserves the
platform message_id. Removed the dead loop, consolidated to a single
content-match branch (renamed 'A: content match'). Branch B (system
note) unchanged. Updated the test name + docstring to reflect this.
Note: self._lock is no longer taken in append_to_transcript (was guarding
the JSONL file append). SQLite append_message handles its own concurrency
via WAL mode, so this is safe; flagging for awareness.
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