Add Timeline Comments plugin

[brsbl]

Summary

• add the Timeline Comments plugin as a self-contained plugins/timeline-comments workspace
• attach durable comment threads to exact user and agent message selections with underlines, nearest-gutter markers, local collision clusters, compact source popovers, and a thread-scoped management panel
• support replies, recoverable edit/reply drafts, deletion, resolve/reopen, contextual individual agent handoff, and bulk current-thread handoff through a fresh mention
• keep agent handoff source-visible and editable while closing the originating popover before navigation
• persist comments in plugin-owned SQLite with schema-validated RPC, bounded pagination, optimistic concurrency, realtime reconciliation, BB-thread lifecycle cleanup, and a read-only CLI
• add accessible focus behavior, consistent body validation, larger compact action targets, the repository card, plugin README, portable SDK declarations, focused tests, and a real-browser interaction harness

Screenshots

Comment from the floating text-selection menu

Anchored gutter thread with a reply

Dependency

This plugin depends on ymichael/bb#846, a separate BB-core PR that adds the 0.0.34 host contracts for selection-only message-action placement, rendered UTF-16 selectors, content-script RPC/realtime/navigation, stable timeline DOM hooks, and thread-panel message reveal. This PR should remain draft until that host work lands.

Validation

• npm run check
• npm run test:browser --workspace=bb-plugin-timeline-comments
• Timeline Comments: 22 focused tests
• real-Chromium coverage for gutter clustering, focus movement/restoration, input validation, contextual handoff, popover cleanup, and narrow-viewport positioning
• live selection, gutter-thread, and reply flows exercised against the BB host branch
• repository hygiene, all workspace typechecks/builds/tests, scaffold smoke, built tests, and artifact validation pass