Codex: serve and delegate A2A tasks

[dimavrem22]

Decisions

• Durability: Write receiver deliveries before acknowledgement and reconcile registry state before paging every submitted task on restart.
• Cross-process authorization: Bridge verified A2A turn context to the Codex MCP subprocess through a private mode-0600 file keyed by the isolated session, then remove it when the turn ends.
• Delegation continuity: Persist outbound message/retry data before SendMessage, promote it to (identity, RPC origin, context) after the server returns, and inject later task updates into the originating Codex session.
• Compatibility: Keep the base SDK range for non-A2A users, while documenting that A2A requires Inkbox SDK 0.5.6 or newer.

Changes

• Add A2A call, check/wait, reply, and explicit inbound complete/ask/fail tools.
• Subscribe and serve inbound A2A tasks with durable deduplication, serialized context sessions, task cancellation, restart recovery, and guarded default completion.
• Persist caller-side delegation routing and return a2a.sent_task.updated events to the delegating session.
• Bump the plugin to 0.1.5.

Related pull requests

• Core: servers PR #395, inkbox PR #123, console PR #210, admin-console PR #120, website PR #151
• Hosts: Hermes PR #72, Claude Code PR #45, OpenCode PR #9, OpenClaw PR #43

Merge order

Merge after backend deployment and the 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- uv run --with pytest python -m pytest -q: 296 passed, 22 skipped.