feat(mcp): give agents description, tags, author, relevance and rehostPolicy

[MyPrototypeWhat]

search_references used to give agents only the id, title, license and links. An agent therefore could not judge whether a result really matches, and it had no way to know whether it may store the result's media. rights.rehostPolicy was validated on every record, but nothing ever read it.

Change

• New fields on each result.
  • description: the source's own caption, capped at 280 characters.
  • tags: the first 8 of the source's subjects.
  • author.
  • relevance: the 0..1 cross-source score after fusion and rerank.
  • rehostPolicy.
• rehostPolicy gets a reader. The output schema describes each value: hotlink-required means link only, cache-allowed means download and cache, thumbnail-only means keep only the thumbnail, and no-store means keep nothing. The tool description also tells the agent to follow it before downloading or storing anything.
• sourceScore is left out on purpose. It uses each source's own scale and only orders results within that source, so an agent comparing it across sources would be misled. relevance is the score an agent can compare.
• Core exports REHOST_POLICIES. Core's own schema now uses it too, so the value list lives in one place, the same pattern as LICENSE_IDS and INTENTS.
• Docs. The root README, the core README and the MCP README are updated. The changeset is minor for @refkit/core (new export) and @refkit/mcp (new output fields).

Testing

• Two new MCP tests, both written first and seen failing.
  • The first checks the projected fields: a long description is truncated with an ellipsis, a short one passes unchanged, tags are trimmed to 8, author and rehostPolicy come through, relevance is in 0..1, and sourceScore is absent.
  • The second checks that the tool description and the output schema both explain the four rehostPolicy values.
• Full gate: pnpm typecheck, pnpm lint, pnpm build, pnpm test:run (521 passed, 23 skipped) and pnpm smoke:artifact.