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/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'