feat(plugin): dashboard-panel contribution axis in the plugin manifest

[rickylabs]

Split from #711 (decision recorded in its worklog + PR #724): the plugin manifest (packages/plugin/src/config/domain/plugin-contributions.ts) has no frontend/dashboard-panel contribution axis — contributions cover services/streams/db/etc. only. This is the framework precondition for the /extensions manager and the 7-member DashboardContribution family in the merged dashboard design (coverage-matrix rows #427/#420).

Scope

• Design the contribution-axis schema (doctrine-first: contract, versioning, quarantine semantics for host/contract version drift).
• Extend the manifest + registry compilation so plugins can declare dashboard panels.
• Do NOT implement dashboard rendering here — this is the manifest/framework seam only.

Acceptance

• Contribution-axis contract designed and recorded (archetype/public surface) before implementation.
• Manifest schema + registry generation support panel contributions with tests.
• plugin info/doctor surface the declared panels.

Refs #711, #400.