refactor(agent): fold agentInterruptReasonOf into loop-private slot invariants

The public classifier existed to defend an exported reader against
arbitrary signals, but its only production caller is the loop reading
its own machine-private turn signal, where cancel() is the sole aborter
and always writes one frozen canonical cause. Delete the export and its
15-line structural validation: settle() states the slot invariant with
one cast, the boolean call sites ask signal.aborted directly, and the
retry veto drops entirely because a requested window already implies a
live signal (cancel() retires the window before aborting).

The abort(reason) channel and first-wins semantics are unchanged; only
the reader's publicness is gone, and with it the paranoia it required.

.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md

@@ -22,7 +22,7 @@ The driver keeps only a cause-less pre-run marker for queued work cancelled befo
 
 The explicit event signatures keep their positional form and place `signal` immediately before a waterfall's final `next`. Prompt submission, request configuration, step-result processing, continuation, and terminal stop join the pre-existing explicit signal seams for pre-step, session prefix, model generation, tool execution, approval, and subagent or workflow requests. Hook bridges must also supply `RunHookOptions.signal`, so a turn cancellation reaches the bash executor's process-group kill and join boundary. `SystemPrompt.assemble()` carries `signal?: AbortSignal` in `AssembleContext` because that object is an explicit request value that can also represent signal-less assembly outside a turn. Listeners may cooperate with the signal but must not retain it to control another turn.
 
-`ctx.agents` continues to carry only the initiating Agent. Ambient Agent presence does not imply liveness, a current turn, or cancellation authority, and `agentInterruptReasonOf(signal)` reads only its explicit argument. Concurrent Agents isolate both their initiator identities and their turn signals; a child driver shadows the parent initiator while its parent request signal still travels through the subagent seam.
+`ctx.agents` continues to carry only the initiating Agent. Ambient Agent presence does not imply liveness, a current turn, or cancellation authority. The cause reader is private to the loop and states the machine-private slot invariant (only `cancel()` aborts a turn controller, always with a canonical frozen cause) instead of re-validating the reason structurally; no public helper reads a cause off an arbitrary signal. Concurrent Agents isolate both their initiator identities and their turn signals; a child driver shadows the parent initiator while its parent request signal still travels through the subagent seam.
 
 Agent disposal requests the runtime-only `{ kind: 'disposed' }` interruption on the active holder. If cancellation already won the controller reason, the reason cannot be rewritten, so terminal classification first checks lifecycle state: disposed wins, then a supported `user` or `parent` cause becomes the coarse aborted outcome, and unrelated exceptions retain the existing error path. ACP cancellation maps to `user`; in-process spawn and fork propagation map to `parent`. Remote ACP subagents retain their existing wire protocol.