feat(codex): wire /goal slash command with mid-turn support

[boudra]

Summary

• Adds optional tryHandleOutOfBand on AgentSession so providers can handle slash commands as side effects, bypassing turn allocation and the activeForegroundTurnId gate. Manager dispatches handler events through recordTimeline + dispatchStream so timeline rows persist.
• Codex provider implements it for /goal (set / pause / resume / clear), gated to codex >= 0.128.0 with --enable goals. Older binaries don't see the command and don't get the launch flag — no crash, no surprise.
• Mid-turn /goal pause now works concurrently with a live turn against the same thread.

Why this shape

/rewind allocates a turnId and trips the same active-turn gate, so it isn't a usable model for mid-turn side effects. The new path is genuinely additive: providers opt in by implementing tryHandleOutOfBand, manager routes around turn machinery only when they do. No existing provider changes, no WS schema changes, fully BC-compatible with old clients.

Test plan

• npm run typecheck clean
• npm run lint clean
• E2E codex-goal-mid-turn.real.e2e.test.ts against real codex 0.128.0 — 3 cases, all pass in ~49s:
  • /goal <obj> sets goal; model reads it back via sentinel on a follow-up turn
  • /goal pause mid-turn during sleep 30 — turn finishes with done, ack persisted
  • /goal clear — ack persisted

Known follow-up (not blocking)

mergeAssistantChunks in timeline-projection.ts:193 collapses contiguous assistant_message rows. When /goal pause lands mid-turn, the ack and the resumed model stream are visually glued together in the default projected view. Test asserts against the canonical projection. UI fix is a separate PR.