Emit proposed plans for plan-capable clients

[s-cork]

Fork-only draft for review. This does not submit or retarget anything in agentclientprotocol/codex-acp.

Summary

• emit completed live plan items as ACP plan_update events when the client advertises Plan support
• replay saved plan items through the same representation
• preserve the existing agent-message behavior for clients without Plan support
• keep completed item text authoritative, with buffered deltas as a fallback

Rationale

agentclientprotocol/codex-acp#326 fixed plan visibility by emitting completed plans as ordinary final-answer messages. That preserves broad client compatibility, but loses the plan semantic: capable clients cannot distinguish a proposed plan from a normal final answer or render it as a first-class plan artifact.

This change preserves that fallback while emitting plan_update only when the client advertises Plan support, including during session replay.

The main review points are capability gating and parity between live events and loaded session history.

Testing

• npm run typecheck
• npx vitest run src/__tests__/CodexACPAgent/load-session.test.ts — includes legacy replay without Plan capability
• npm test — 329 passed, 28 skipped
• npm run bundle:all with Bun 1.3.11
• git diff --check main...HEAD

Related work

• follow-up to agentclientprotocol/codex-acp#326

AI assisted with implementation, test verification, and PR text.