refactor: type AgentInstance identifiers as UUIDs

[EItanya]

Summary

• represent AgentInstance, share, checkpoint, and A2A context identifiers as uuid.UUID in the database layer
• migrate the corresponding PostgreSQL columns from TEXT to native UUID columns
• generate UUID-typed SQLC models and query parameters, converting only at protobuf/service boundaries
• preserve empty pagination cursors while comparing non-empty cursors as UUIDs
• update database tests to use valid UUID fixtures

Testing

• go test ./...
• make -C go lint

[EItanya]

Drafted this as I'm not convinced it's worth it