Full-history conversation outline for the thread TOC minimap

[brsbl]

What & why

The table-of-contents minimap previously listed only the currently-loaded timeline window, so on a long thread it was incomplete until you scrolled older pages in. This makes it reflect the whole thread, and lets any entry jump to its message — auto-loading older pages as needed.

Builds on the recently-landed right-centered minimap + min-3-user-messages gating + near-bottom active detection (kept as-is).

How

• New GET /threads/:id/conversation-outline — a lightweight list of every user/agent message (id, role, clamped preview, attachment counts). It reuses the same buildThreadTimelineFromEvents projection as the timeline over the full event set, so each outline item's id is identical to its timeline row's id. That identity is what makes scroll-spy highlighting and click-to-jump work. The handler memoizes per (thread, maxSeq); the client query is invalidated by the same realtime events-appended signal as the timeline window.
• Minimap renders from the outline (falling back to the loaded window until it loads). Clicking an entry whose message isn't paginated in yet auto-paginates older pages until the row appears, then scrolls to it — bounded by a hard cap, resilient to pagination errors, and stops if you switch threads mid-jump. The tick rail is bounded + scrollable so long threads don't overflow.

Testing

• Full-repo typecheck: all packages pass. Lint clean.
• @bb/app: 1062 tests pass (incl. a new ThreadTableOfContents jsdom suite covering full-history rendering, attachment-only labels, jump-to-loaded, auto-paginate-to-unloaded, and the no-older-pages case — merged with the existing findActiveItemIds scroll-spy tests).
• @bb/server: 800 tests pass (incl. a new conversation-outline integration test: a segmentLimit=1 page shows only the last turn while the outline returns all messages, ids match the timeline window, plus empty-thread and attachment-only-without-path-leak cases).

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