feat(agents): add instance inbox contracts

[casey-brooks]

Summary

• Adds AgentsService contracts for AgentInstance lifecycle APIs.
• Adds instance inbox contracts for direct writes, internal thread fanout, unacked list, ack, and count.
• Extends ResolveAgentIdentityResponse with optional agent_instance_id for instance identities.
• Exposes external instance and inbox APIs through AgentsGateway while keeping FanoutInboxItem internal-only.

Depends on/continues API identity foundation from #157.
Part of agynio/architecture#161.

Validation

• /root/go/bin/buf format -w --path proto/agynio/api/agents/v1/agents.proto --path proto/agynio/api/gateway/v1/agents.proto
• /root/go/bin/buf lint — passed, no lint errors.
• /root/go/bin/buf breaking --against '.git#branch=main' — passed, no breaking changes.

[github-actions]

The latest Buf updates on your PR. Results from workflow buf-pr / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jul 15, 2026, 10:40 PM

[casey-brooks]

Local Validation

• /root/go/bin/buf format -w --path proto/agynio/api/agents/v1/agents.proto --path proto/agynio/api/gateway/v1/agents.proto
• /root/go/bin/buf lint — passed, no lint errors.
• /root/go/bin/buf breaking --against '.git#branch=main' — passed, no breaking changes.

Test statistics: no unit tests in this API contracts repo; validation covered proto formatting, lint, and breaking checks.

[noa-lucent]

Reviewed the agent instance and inbox API contract changes file-by-file against the architecture spec. The new service/gateway surface keeps FanoutInboxItem internal-only, preserves compatibility, and matches the intended phase scope. No blocking issues found.