[yarolegovich]

• Added a2a catalog add/list/update/remove/search (experimental). They read and write the store shared with ai-catalog-cli (~/.ai-catalog, respecting AI_CATALOG_CACHE_DIR), so a catalog registered with either tool is visible to both, and search lists only A2A agents (application/a2a-agent-card+json).
• Embedded ai-catalog-go v0.3.0 for the catalog model rather than shelling out to the ai-catalog binary. Store I/O lives in the new internal/aicatalog package and keeps every registry field it doesn't own, and its tests read a store written by ai-catalog-cli v0.4.0.
• -a urn:... looks the agent up across the registered catalogs, and --catalog <name|url> narrows the lookup to one registered catalog or an unregistered catalog URL. An identifier listed by more than one catalog is an error that suggests --catalog, and a missing one lists the agents that are in scope.
• -a <url> that serves an AI Catalog instead of a card resolves to the catalog's only A2A agent, and errors with the list of agents otherwise.
• a2a warns about a store whose format it doesn't recognize, reads it on a best-effort basis, and refuses to modify it, since the layout is not part of the spec and ai-catalog is pre-1.0.

-a values starting with urn: used to be treated as hosts and failed, and an existing file named urn:... is still read as a card, so existing invocations are unaffected. Lookups in registered catalogs read only the local store; a2a catalog update fetches them again.

Fixes #19