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.

docs/cordis-catalog/services.md

@@ -216,7 +216,7 @@ roots(): Agent[]
 
 Types: [Agent](../core-data-structures/core.md) · [SessionId](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/index.ts:221`](../../packages/core/agent/src/index.ts)
+Source: [`packages/core/agent/src/index.ts:220`](../../packages/core/agent/src/index.ts)
 
 ## `ctx.approval` — `ApprovalService`