[Issacboy123]

Summary

Controller cancellation could reach an attached child before agent_start without reaching the child's prompt preflight, leaving interruption or shutdown waiting indefinitely. This PR passes the controller run signal into the child prompt and coordinates cancellation across queued, startup-publication, and running states.

Reasoning

Reusing the controller's existing run signal closes the startup gap without adding a second cancellation protocol. Serializing each run's cancellation prevents the prompt signal and session abort paths from aborting the same child twice during agent_start publication. Clearing only a rejected cancellation attempt permits later shutdown or navigation to retry after a transient failure.

Changes

• Passed each controller run's abort signal to AgentSession.prompt() for the child run.
• Centralized run cancellation ownership across interrupts, quiescing, reset, and shutdown.
• Prevented duplicate aborts during the window where the session is streaming but the controller has not published the running state.
• Allowed a later cancellation attempt after an earlier session abort rejects, while sharing concurrent attempts for the same run.
• Added regressions for pre-start interruption, startup publication races, active shutdown, and cancellation retry after transient quiesce failure.

Testing

• node ../../node_modules/vitest/dist/cli.js --run test/suite/subagent-startup.test.ts test/suite/subagent-persistence.test.ts
• node ../../node_modules/vitest/dist/cli.js --run test/suite/agent-session-prompt.test.ts test/suite/subagent-services.test.ts test/suite/subagent-registration.test.ts test/suite/subagent-coordination.test.ts test/suite/subagent-parent-messaging.test.ts test/subagent-controller.test.ts
• npm run check