Stage Browser captures as inspectable mentions

[brsbl]

Summary

• stage one stable inspectable mention per captured selection instead of blockquotes or file attachments
• keep comments as ordinary editable composer text and make each mention independently removable
• resolve immutable DOM, React, accessibility, and geometry metadata only at send time
• keep screenshots exclusively in the inspector preview and exclude them from agent context
• vendor a reproducible compatibility SDK overlay for the repository's unshipped 0.5.0 surface

Verification

• repository hygiene passed across all six plugins
• Browser Context: typecheck, build, and 18 tests
• exact Electron selection, comment, staging, inspector, send, and timeline-reopen flow
• Browser Context, hygiene, and inventory CI jobs pass

BB-Thread-ID: thr_p9qmfkggwm

[brsbl]

Follow-up integration update:

• Browser Context now consumes inspection V2 with explicit scanTruncated handling.
• The 0.5.0 compatibility overlay was rebuilt from core source commit 86def11c2bb7e40be4d052e5173e102fbb932b3f and revendored reproducibly.
• local repository hygiene passes
• Browser Context: 3 files / 18 tests
• CI Browser Context, hygiene, and inventory jobs pass

The remaining red plugin jobs are unrelated generated-CSS/test drift in other plugin workspaces and were already present before this revendor; this focused branch does not rewrite those plugins beyond their required byte-equal SDK declarations.

[brsbl]

Closing because this PR targets the obsolete specialized Browser inspection contract and obsolete SDK provenance. Replacement Browser work will use the content-script runtime and client/window/tab broker architecture; the branch and history remain recoverable.

[brsbl]

Reopened: this Browser work remains active and is intentionally parked until the replacement bb-core content-script runtime and client/window/tab broker APIs are frozen. Do not merge or review against the obsolete contract in the meantime.

[brsbl]

Superseded by the exact-target Browser runtime consumer in #72 and its get-bb/bb core dependency stack #1869 → #1870. Preserving this PR unmerged for history.