Steering should support a host-owned idle fallback

[fontanierh]

Problem

_session/steering returns startedNewTurn when the active turn wins the idle race. That turn runs after the steering request completes, so the host never owns its terminal session/prompt response.

steer while busy -> injected -> original prompt owns completion
steer after race -> startedNewTurn -> no host-owned completion

Proposed compatible behavior

Honor the opt-in used by claude-agent-acp in #919:

{"_meta":{"steering":{"idleBehavior":"promptRequired"}}}

On the idle path, consume nothing and return:

{"outcome":"promptRequired","reason":"noRunningTurn"}

The host can then submit the unchanged input through normal session/prompt. Keep startedNewTurn as the default for compatibility.