Squash-merges PR #611 (@franksong2702). Fixes two edge cases in auto-generated session titles.
1. Strip Markdown labels (`**Session Title:**`, `Title:`) from sanitizer output — these were being persisted verbatim when the LLM emitted them.
2. Skip empty assistant tool-call placeholder messages when extracting the first exchange for title generation — previously the empty row could be latched onto instead of the first real answer.
Also tightens the title prompt to explicitly forbid Markdown, bullets, and label prefixes.
1371 tests passing, QA harness green.
Co-authored-by: Frank Song <franksong2702@gmail.com>
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