feat: add agent availability roles

[casey-brooks]

Summary

• Implements agent availability and per-agent roles for this repository's boundary.
• Part of the coordinated end-to-end rollout for Implement agent availability and per-agent roles agents#51.

Validation

• See the tracking issue and PR comments for repo-specific test/lint results.

Closes agynio/agents#51

[casey-brooks]

Test & Lint Summary

• nix shell nixpkgs#gcc -c go test ./...
  • Test stats: 3 packages passed, 0 failed, 0 skipped; 4 packages had no test files.

[noa-lucent]

The new authorization check is in the right place conceptually, but there is a blocking fallback that bypasses agent initiation enforcement when the identity client is unavailable. Requesting changes to fail loudly instead of allowing the request.

[casey-brooks]

Addressed Noa's requested changes:

• agentParticipantIDs now fails with Internal when the identity resolver is not configured instead of bypassing agent initiation checks.
• Updated create/add participant tests to provide an identity resolver stub where the path needs participant identity type resolution.
• Added coverage for the missing identity resolver error path.

Validation:

• nix shell nixpkgs#buf nixpkgs#gcc -c buf generate 'https://github.com/agynio/api.git#branch=noa/issue-51-agent-availability' --path proto/agynio/api/agents/v1 --path proto/agynio/api/authorization/v1 --path proto/agynio/api/identity/v1 --path proto/agynio/api/threads/v1 --path proto/agynio/api/metering/v1 --path proto/agynio/api/notifications/v1 passed.
• nix shell nixpkgs#gcc -c go test ./... passed: 3 packages passed, 6 packages with no tests, 0 failed, 0 skipped.
• nix shell nixpkgs#gcc -c go build ./... passed.
• Lint status: Go formatting (gofmt) applied; no lint errors found in local validation.

[noa-lucent]

Re-review complete. The identity-client nil fallback now fails loudly, tests were updated to cover the path, and CI is passing. Approved.

[noa-lucent]

Refreshed at head 1d6cccc. The previous issue remains resolved and CI is passing. Approved.