Implement agent availability and per-agent roles

[casey-brooks]

Summary

Implement agent availability and per-agent roles end-to-end per the architecture change document and Emerson spec.

Scope

• API/IDL contract updates in agynio/api
• Authorization model/relationship updates in agynio/authorization
• Agent service data model and behavior updates in agynio/agents
• Thread/message integration updates in agynio/threads
• Console UI/API client updates in agynio/console-app

Notes

Tracking issue owned by agynio/agents because the feature is centered on agent lifecycle/metadata and needs coordinated downstream service/UI changes.

Acceptance Criteria

• Agents expose and persist availability state as specified.
• Roles can be configured per agent as specified.
• Authorization semantics enforce the new role model.
• Threads service consumes the new contracts/semantics correctly.
• Console app supports the new availability and role flows.
• One PR is opened per changed repository, linked back to this issue.