feat(mcp): accept a human-readable coordinate for the released_prompt contract

[kl3inIT]

Problem

released_prompt takes asset_id and release_id as UUIDs. MCP completion (#121) inserts the literal argument value, so the picker a host renders is a list of UUIDs. Filtering by title/slug works, but the list itself is unreadable.

Proposal

Additive, non-breaking:

• keep asset_id / release_id, make them optional;
• add optional namespace, slug, version;
• require exactly one complete form, fail closed otherwise;
• completion then returns readable values for the coordinate arguments.

Precedent in this repository

resolve_skill already does exactly this: namespace + slug + version resolved through /api/asset-delivery/skills/{namespace}/{slug}/versions/{version}/manifest. Prompts have no equivalent resolver yet, so the API needs one.

Pinning is not weakened: resolve_skill requires an exact version label and has no latest. The coordinate form must keep that rule.

Acceptance

• Prompt coordinate resolver in the API + contracts/openapi.json regenerated
• released_prompt accepts either form, exactly one, fail closed
• Completion returns readable coordinate values
• Existing UUID clients unaffected — covered by a test
• assistant-and-mcp spec + test matrix reconciled

Depends on

The release-history endpoint, which supplies the exact version labels this contract needs.