OpenCode: serve and delegate A2A tasks

[dimavrem22]

Decisions

• Durability: Store each receiver delivery before acknowledgement, reconcile the registry first on restart, and then page all submitted tasks.
• Session isolation: Serialize inbound work in a2a:{identity}:{context} sessions and authorize complete, ask-caller, and fail through active turn context only.
• Delegation continuity: Persist outbound message/retry data before SendMessage, promote it to (identity, RPC origin, context), and inject later task updates into the original OpenCode session.
• Write controls: Reuse the existing outbound approval and recipient allowlist for A2A calls and replies.
• Compatibility: Keep the base SDK range for non-A2A users, while documenting that A2A requires @inkbox/sdk 0.5.6 or newer.

Changes

• Add default-enabled A2A call, check/wait, reply, and inbound outcome tools.
• Add durable inbound serving, task-addressed cancellation, context serialization, restart recovery, and guarded default completion.
• Add durable caller-side delegation routing and sent-task update injection.
• Document the A2A tool group and bump the plugin to 0.1.1.

Related pull requests

• SDK/CLI: Inkbox SDK PR #123
• Hosts: Hermes PR #72, Claude Code PR #45, Codex PR #26, OpenClaw PR #43

Merge order

Merge after the paused-contract base PR, backend deployment, and SDK 0.5.6 release.

Verification\n\n- Staged rollout remains compatible with an API that does not yet accept A2A webhook event types; legacy identity events continue and full A2A reconciliation retries on the next startup.\n- npm run lint

• npm run typecheck
• npm test: 640 passed.
• npm run build