feat(core): make turn cancellation explicit

packages/core/agent-loop/src/cancellation.ts

@@ -0,0 +1,31 @@
+/** Turn-scoped cancellation ownership for the concrete AgentLoop driver. @module dsh-agent-loop/cancellation */
+
+import type { AgentCancelCause } from '@deepseek-ai/dsh-agent'
+
+/** Stable runtime-only reason used when lifecycle teardown interrupts a turn. */
+export const DISPOSED_INTERRUPT_REASON = Object.freeze({ kind: 'disposed' } as const)
+
+/**
+ * Owns the single controller shared by every asynchronous boundary of one turn.
+ * The first request wins because a later caller must not rewrite the cause
+ * observed by earlier listeners.
+ */
+export class TurnCancellation {
+  readonly #controller = new AbortController()
+
+  /** The explicit signal passed through this turn's execution boundaries. */
+  get signal(): AbortSignal {
+    return this.#controller.signal
+  }
+
+  /**
+   * Abort the turn once.
+   * @param reason - a validated caller cause or lifecycle disposal marker.
+   * @returns whether this request established the signal reason.
+   */
+  request(reason: AgentCancelCause | typeof DISPOSED_INTERRUPT_REASON): boolean {
+    if (this.signal.aborted) return false
+    this.#controller.abort(reason)
+    return true
+  }
+}