Add Browser Context runtime consumer

[brsbl]

Summary

Adds Browser Context as the first consumer of bb's general Browser page-script runtime. Users can select page elements or regions, attach editable comments, stage inspectable composer mentions, and explicitly enable exact-target agent control from the Browser toolbar.

Agent control and persisted state are keyed by the complete Browser identity: clientId + windowId + tabId + navigationEpoch. Realtime reconnects reconcile missed state without letting stale hydration overwrite newer updates. Screenshots remain preview-only; resolved mention context contains the immutable bounded DOM, React, accessibility, and geometry snapshot.

Depends on get-bb/bb#1869 at dd66e0893 and get-bb/bb#1870 at c3fe8ee68. Only this plugin opts into the exact @get-bb/plugin-sdk@0.4.9 and matching builder artifacts; the repository default remains unchanged for other plugins.

Visual evidence

All captures use the exact core top c3fe8ee68, the same Browser Core Test Lab fixture, and a 1280×900 desktop dev-app window. The before state has Browser Context disabled; every after state comes from the exact PR head e5628940b.

Before — Browser Context disabled; no contributed Browser controls

After — plugin enabled with agent-control and selection actions in the native toolbar

After — exact-target agent control active with the scoped notification in-thread

After — inspectable captured selection with its screenshot and two comments

Validation

• Browser Context check: typecheck, build, 41/41 tests
• Selection activation regression: 4/4 focused tests
• Repository hygiene: 8 plugins
• Focused packed-artifact validation: 10 files
• Full install-ref generation and rebuild validation: passed for all plugins
• Scaffold clean-install smoke: passed after npm ci
• Real Electron QA: user and agent entry, exact-target notification, exit, navigation epoch invalidation, reconnect/hydration behavior, click selection without page activation, two sequential comments, mention staging, tooltip, and inspector reopening

Frozen provenance

• Core top source: c3fe8ee68c5ddfe80b58e4b60bcf4da49bc1e43b
• @get-bb/plugin-sdk@0.4.9: 69a691ae3055be3a416c36eb4588f6e286b265d7687dcd259fc2d1c13b3a91e3
• bb plugin build@0.39.0: 4228738add52171fb1adb7ffa8768a694066cfe2a761c134d2a919640e5383f6

BB-Thread-ID: thr_p9qmfkggwm