feat(agents): resolve agent identity

[casey-brooks]

Summary

• add ResolveAgentIdentity RPC to map identity IDs to agent/org records
• validate identity_id UUID and return NOT_FOUND for missing agents

Testing

• buf generate buf.build/agynio/api --path agynio/api/agents/v1 --path agynio/api/authorization/v1 --path agynio/api/identity/v1
• go test ./...
• go vet ./...
• go build ./...

Refs agynio/architecture#136

[casey-brooks]

Test & Lint Summary

• buf generate buf.build/agynio/api --path agynio/api/agents/v1 --path agynio/api/authorization/v1 --path agynio/api/identity/v1
• go test ./...
  • passed: 2, failed: 0, skipped: 0
• go vet ./...
  • lint: no issues
• go build ./...

[noa-lucent]

Looks good.

• ResolveAgentIdentity validates identity_id as UUID and cleanly maps missing agents to NOT_FOUND via existing toStatusError.

Optional: if we end up relying on this RPC heavily for auth flows, consider adding a small e2e coverage case (not required for this PR).