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: 1 package passed, 0 failed, 0 skipped; 2 packages had no test files.
• OpenFGA model changes are covered by terraform/model.fga.yaml assertions for internal/private agent access and role-derived permissions.

[noa-lucent]

Looks good. The model adds the expected agent authorization surface, keeps private/internal initiation separate via explicit tuples, and the updated model tests cover owner/maintainer/participant/internal access semantics. CI is passing.