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/agent-loop/src/loop.ts

@@ -116,6 +116,14 @@ export interface LoopHandle {
    * marker governs exactly one cancellation and never leaks to a later prompt.
    */
   isCancelled(): boolean
+  /**
+   * The resolved reason for the pending cancel (`reason ?? 'cancelled'`), read
+   * by the marker branches (pre-step / continuation) so a turn dropped where no
+   * `AbortController` carries the reason still records the caller's
+   * `cancel(reason)` value — matching the mid-step abort path. Only meaningful
+   * when {@link isCancelled} is true.
+   */
+  cancelReason(): string
   /** Clear the cancel marker (called once per iteration after the turn returns). */
   clearCancel(): void
   /**
@@ -396,7 +404,7 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
       // already-appended step/start.
       if (handle.isCancelled()) {
         handle.setAbort(undefined)
-        reason = { kind: 'aborted', reason: 'cancelled' }
+        reason = { kind: 'aborted', reason: handle.cancelReason() }
         closeStep()
         break
       }
@@ -466,7 +474,7 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
       // ends the turn here. cancel() also cleared the steering FIFO, so the
       // override above did not re-arm continuation.
       if (handle.isCancelled()) {
-        reason = { kind: 'aborted', reason: 'cancelled' }
+        reason = { kind: 'aborted', reason: handle.cancelReason() }
         break
       }