Add Browser Context selection plugin

[brsbl]

Summary

• add the Browser Context plugin and its Browser navigation action
• support click and drag-region capture with comments and a movable review surface
• add deterministic, bounded page/DOM metadata and focused plugin coverage

Verification

• Browser Context workspace check
• real Electron click, drag, cancellation, responsive layout, and composer QA performed in the originating thread

BB-Thread-ID: thr_p9qmfkggwm

[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.