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/tests/cancel.spec.ts

@@ -186,9 +186,11 @@ describe('Agent.cancel()', () => {
     await waitForIdle(ctx, agent)
     dispose()
 
-    // No step streamed (the model never ran), and the turn ended aborted.
+    // No step streamed (the model never ran), and the turn ended aborted with
+    // the CALLER's reason — the marker carries `cancel(reason)` through even
+    // though no AbortController observed it in this window.
     expect(streamed).toBe(false)
-    expect(reasons).toEqual([{ kind: 'aborted', reason: 'cancelled' }])
+    expect(reasons).toEqual([{ kind: 'aborted', reason: 'from turn-start' }])
   })
 
   it('cancel during the continuation window ends the turn aborted and runs no further step', async () => {
@@ -219,9 +221,10 @@ describe('Agent.cancel()', () => {
     await waitForIdle(ctx, agent)
 
     // Only ONE step ran (the second was cancelled in the continuation window),
-    // and the turn ended aborted.
+    // and the turn ended aborted with the CALLER's reason (carried by the
+    // marker, since the finished step's AbortController was already cleared).
     expect(steps).toBe(1)
-    expect(reasons).toEqual([{ kind: 'aborted', reason: 'cancelled' }])
+    expect(reasons).toEqual([{ kind: 'aborted', reason: 'from continuation' }])
   })
 
   it('cancel from a synchronous agent/status(running) listener drops the turn (window 2)', async () => {