Define plugin discovery contracts

[brsbl]

Product intent and requirements

• Give plugin discovery one shared contract across the app, server, CLI, SDK, and database instead of allowing each surface to invent its own shapes.
• Keep the category taxonomy closed and stable so the same plugin has the same category everywhere.
• Preserve the released strict marketplace v1 contract; richer v2 metadata must be additive and backward compatible.
• Model authorship, listing lifecycle, screenshots, discovery metadata, and health explicitly without changing the host-daemon protocol.

What was wrong

Plugin discovery, marketplace metadata, capture state, category identity, and authored-listing lifecycle were represented by overlapping local shapes. That left no single compatibility boundary for the server, SDK, CLI, database, and UI layers that consume the same concepts.

What changed

• Adds shared domain schemas for the closed plugin taxonomy, marketplace v1/v2 entries, discovery metadata, screenshot capture, surface detection, and listing lifecycle.
• Extends the server and public SDK contracts with catalog metadata and authored-listing operations.
• Documents the card taxonomy and updates the Plugin SDK version metadata.
• Keeps marketplace v1 as a distinct strict schema while defining v2 additively; this layer does not change the host-daemon protocol.

Product context

This contract layer enables the category-led Browse experience shown below in the later UI layer; it does not itself change rendered UI.

## How you verified

• This contract-only layer has no intentional rendered UI change, so before/after UI evidence is not applicable.
• Pull-request CI is green on exact head 35999318eb61728e98e7798afc2ffb5436833d65; all required checks passed and the platform-inapplicable jobs were skipped.
• No CI-equivalent tests, typechecks, or lint were run locally per repository policy. Pull-request CI is the validation path.

BB-Thread-ID: thr_9tmqez22hd

AGENT GENERATED

[SawyerHood]

Closing: the discovery contracts landed in a rebuilt form in #2880 (marketplace v2 contracts and parser, with marketplace-defined categories and tolerant v2 parsing). Thank you for the groundwork; the domain schemas and tests were ported from this PR with credit.

AGENT GENERATED