Why

The goal extension needs a way to resume an active goal after the thread becomes idle, but the old core goal runtime should not be refactored as part of this step. The missing piece is a small core-owned turn-start primitive: let an extension ask for a normal model turn only when the thread is idle, and otherwise fail without injecting into whatever is currently active.

What Changed

• Adds CodexThread::try_start_turn_if_idle(...) as the narrow extension-facing primitive for synthetic idle work.
• Implements the session side so it refuses to start when:
  • the provided input is empty,
  • the session is in plan mode,
  • a turn is already active, or
  • trigger-turn mailbox work is pending.
• Gives trigger-turn mailbox work priority if it appears while the idle turn is being prepared.
• Wires GoalExtension::on_thread_idle to read the active persisted goal and submit the continuation prompt through this idle-only primitive.
• Keeps the legacy core goal continuation implementation in place instead of folding it into this PR.

Behavior

This is intentionally best-effort. If try_start_turn_if_idle observes that the thread is not idle, or that higher-priority mailbox work should run first, it returns the input to the caller. The goal extension drops that continuation prompt and waits for a future idle opportunity instead of injecting stale synthetic goal text into an active turn.

Validation

• just test -p codex-core try_start_turn_if_idle_rejects_active_turn_without_injecting
• just test -p codex-goal-extension