fix(agent): carry cancel(reason) through the marker-only windows (review)

A reviewer found that `cancel(reason)` only preserved the caller's reason when
an active AbortController observed it (the mid-step path, via
`abort.signal.reason`). The marker-only windows (step-start at loop.ts and the
continuation gate) hardcoded `reason: 'cancelled'`, so the logged `turn/end`
reason was race-dependent on WHERE the cancel landed and the public
`cancel(reason?)` parameter was half-effective.

Capture the resolved reason (`reason ?? 'cancelled'`) on the agent when the
marker is armed, expose it on the LoopHandle as `cancelReason()`, and use it in
both marker branches so a turn dropped without a live controller records the
SAME `{kind:'aborted', reason}` the mid-step path produces.

The two existing window tests asserted `reason: 'cancelled'` while passing
`'from turn-start'` / `'from continuation'` — they documented the bug. Updated
both to assert the caller's reason (behavior + test changed together, per
AGENTS.md "tests document behavior, not golden truth").

Also fixes two stale docs the PR's contract change left behind: the
module-level ACP mapping comment and `codec.ts` both still said `session/cancel
-> agent.abort()`.

packages/acp/src/codec.ts

@@ -26,7 +26,7 @@ import type { ContentBlock as AcpContentBlock, StopReason } from '@agentclientpr
  *
  * - `completed` → `end_turn` (the model chose to stop)
  * - `max-tokens` → `max_tokens` (cut off at the output-token ceiling)
- * - `aborted`   → `cancelled` (an `agent.abort()`, e.g. from `session/cancel`)
+ * - `aborted`   → `cancelled` (a step abort or a queue-aware `agent.cancel()`, e.g. from `session/cancel`)
  * - `error`     → `end_turn` (defensive fallback only: the bridge REJECTS the
  *                 `session/prompt` RPC on an error turn BEFORE calling this, so
  *                 a client sees a JSON-RPC error, not a stop reason — see

packages/acp/src/index.ts

@@ -13,7 +13,9 @@
  * - `session/load`   → `ctx.agents.resume(...)` then replay the event log
  * - `session/prompt` → `agent.send()`, settle on the owning turn's end (a turn
  *                      that ends in `error` rejects the RPC)
- * - `session/cancel` → `agent.abort()` + settle the in-flight prompt
+ * - `session/cancel` → `agent.cancel()` (the queue-aware cancel: aborts a
+ *                      running step, clears queued + steering work, and drops a
+ *                      turn about to start) + settle the in-flight prompt
  *
  * Multi-session (RFC 011): N concurrent sessions per connection, each mapped to
  * its own `ReactLoopAgent`. Sessions are keyed by id in `sessions` (forward) with an