A user reported a stuck-task pattern: the plan card freezes at `0/N
done`, can't be dismissed, and `/new` doesn't clear it either.

Reproducer

The model proposes a plan, the user approves, the model calls a tool
that goes through pauseGate (e.g. `mark_step_complete` →
`plan_checkpoint`). Either the modal mounts and the user closes the
terminal modal in some way that misses the resolve, or the modal mounts
but the user wants to bail out via Esc / `/new`.

Root cause

`PauseGate.ask()` returns a Promise that ignores AbortSignal — it only
resolves when someone calls `gate.resolve(id, verdict)`. Three knock-on
problems:

1. Esc during busy runs `loop.abort()`, which aborts the turn's
   AbortController. The model fetch path bails on `signal.aborted`,
   but the `await pauseGate.ask(...)` inside a tool fn doesn't see
   the signal — the tool fn stays awaiting forever.
2. With the tool fn stuck, the loop's `for-await` stays awaiting,
   `busy` stays `true`, and `` stays
   disabled. The user can't type.
3. Even when the user does manage to type `/new`, `handleSubmit`'s
   `if (busy) return` (App.tsx:2066) silently drops it.

The plan card the user sees frozen at `0/N done` is the visible
manifestation of the awaiting tool fn behind it.

Fix

• Add `pauseGate.cancelAll()` that resolves every outstanding request
  with its kind's safe-cancel verdict (`run_command` → `deny`,
  `plan_proposed` → `cancel`, `plan_checkpoint` → `stop`,
  `plan_revision` → `cancelled`, `choice` → `cancel`). Awaiting tool
  fns return or throw cleanly.
• Add a `resetPendingModals` helper in App.tsx that clears every
  `setPending*` modal state and calls `pauseGate.cancelAll()`.
• Wire it into:
  • Esc-during-busy — replaces the prior edit-review-only cleanup.
  • `/new` — `apply-slash-result`'s `result.clear` branch now
    calls it before `log.reset()`, so the wipe is atomic.

After the fix:

• Esc during a stuck plan_checkpoint flushes the gate → the tool's
  await returns with `{type: "stop"}` → `mark_step_complete` throws
  → the loop's `signal.aborted` check catches the next iter → busy
  clears → prompt re-enables.
• `/new` (issued after Esc, since it still respects the busy gate)
  now also flushes any modals/gate state that survived a partial
  cleanup.

Test plan

• `npx tsc --noEmit` clean
• `npx vitest run` — 2406 passed, 2 skipped (added 2 cancelAll
  tests in `tests/pause-gate.test.ts`)
• Smoke: start a plan, when the checkpoint modal mounts hit Esc —
  confirm modal closes, prompt re-enables, busy spinner stops
• Smoke: same scenario but type `/new` after Esc — confirm cards
  wipe and no plan card returns