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.

packages/core/agent/src/cancellation.ts

@@ -1,30 +0,0 @@
-/** Runtime reason inspection for explicit turn cancellation. @module @deepseek-ai/dsh-agent/cancellation */
-
-import type { AgentInterruptReason } from './types.ts'
-
-/**
- * Read a supported agent interruption from an explicitly supplied signal.
- * Unknown reasons return `undefined`; ambient initiator identity does not grant
- * cancellation authority.
- * @param signal - the current turn's explicit control signal.
- * @returns its canonical reason, or `undefined` while live or unsupported.
- */
-export function agentInterruptReasonOf(signal: AbortSignal): AgentInterruptReason | undefined {
-  if (!signal.aborted) return undefined
-  const reason: unknown = signal.reason
-  if (typeof reason !== 'object' || reason === null || Array.isArray(reason)) return undefined
-  const prototype = Object.getPrototypeOf(reason) as unknown
-  const keys = Reflect.ownKeys(reason)
-  if ((prototype !== Object.prototype && prototype !== null)
-    || keys.length !== 1 || keys[0] !== 'kind') return undefined
-  switch ((reason as { readonly kind?: unknown }).kind) {
-    case 'user':
-      return Object.freeze({ kind: 'user' })
-    case 'parent':
-      return Object.freeze({ kind: 'parent' })
-    case 'disposed':
-      return Object.freeze({ kind: 'disposed' })
-    default:
-      return undefined
-  }
-}

packages/core/agent/src/index.ts

@@ -15,7 +15,6 @@ import type { SessionEvent, SessionId } from '@deepseek-ai/dsh-session'
 import type { Agent, AgentOptions } from './types.ts'
 
 export * from './types.ts'
-export { agentInterruptReasonOf } from './cancellation.ts'
 export * from './llm-target.ts'
 export { agentCarrier, agentEvents, assembleContextFor, emitAgentEvent } from './dispatch.ts'
 export type { AgentEventDispatch, AgentSubjectEvent } from './dispatch.ts'