fix: keep idle steering fallback host-owned

[fontanierh]

Lifecycle

active turn  -> inject                 -> original session/prompt completes
idle + opt-in -> promptRequired        -> host starts owned session/prompt
idle default  -> startedNewTurn        -> existing behavior

Adds the promptRequired opt-in already shipped by claude-agent-acp in #919. Invalid idle policies fail closed.

Tests cover idle, the completion race, legacy behavior, and malformed metadata.

Fixes #440.