feat: add prompt-required idle steering fallback

[djson9]

Summary

• add an opt-in _meta.steering.idleBehavior: "promptRequired" extension contract
• return {outcome: "promptRequired", reason: "noRunningTurn"} when no turn is active or the tracked turn ends during injection
• preserve the existing startedNewTurn behavior when the host does not opt in
• reject unsupported idle behavior values

Why

Hosts with a durable prompt queue need to own the fallback when steering loses a race with turn completion. Starting an adapter-owned turn after the original session/prompt lifecycle has ended can detach output from its owning request. This opt-in matches the host-owned fallback already used by Claude ACP and is compatible with the no-running-turn direction in agentclientprotocol/agent-client-protocol#1261.

Verification

• npm run typecheck
• steering tests: 10 passed
• full suite: 459 passed, 28 skipped
• npm run build
• real Codex active-turn demo: steering returned injected, changed execution, original prompt ended with end_turn
• real Codex idle probe: returned promptRequired/noRunningTurn and emitted no detached output