Codex: serve and delegate A2A tasks

[dimavrem22]

Decisions

• Live protocol coverage: Exercise inbound and outbound behavior at both single-turn and multi-turn lengths. Serialize the four cases because they share live identities and one tunnel endpoint.
• 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 a published-SDK live A2A workflow that boots the real host and verifies all four protocol legs.
• 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

• Hermes PR 75
• Claude Code PR 45
• OpenCode PR 9
• OpenClaw PR 43

Prerequisites

Merge after A2A server support is deployed and Inkbox SDK 0.5.6 is available.

Verification

• uv run --with pytest python -m pytest -q: 310 passed, 22 skipped.
• Unit and host-contract checks passed.
• Full-stack live checks passed, including mock and real channels, inbound and outbound A2A at single-turn and multi-turn lengths, voice, external events, and the aggregate gate.