[Issacboy123]

Summary

Child sessions do not load parent extensions, so spawning a child could bypass execution-mutating hooks registered by the root session. This PR makes controller-bound spawning fail before child creation whenever the current parent extension runner contains an incompatible hook.

Reasoning

The root session remains the authority for extension policy, while the controller owns child admission. Passing a private callback between those boundaries lets every spawn inspect the current extension runner, including after reloads, without exposing runner internals or copying parent extensions into children. Checking both tool entry and controller admission prevents side effects before rejection while preserving unbound programmatic controllers and observation-only hooks.

Changes

• Added deterministic detection for registered resources_discover, input, context, before_agent_start, before_provider_request, before_provider_headers, session_before_compact, tool_call, tool_result, and message_end hooks.
• Rejected incompatible tool-driven spawns before argument validation, provenance capture, child session creation, reservation, persistence, or provider work.
• Rechecked parent policy in bound controller spawning while leaving unbound controller spawning and non-spawn child actions unchanged.
• Added focused coverage for every incompatible hook, ordered combinations, reload-time policy changes, observation-only hooks, side-effect prevention, and unbound controllers.
• Documented the fail-closed spawning rule in the SDK and usage guides and recorded the fix in the changelog.

Testing

• node ../../node_modules/vitest/dist/cli.js --run test/suite/subagent-registration.test.ts
• node ../../node_modules/vitest/dist/cli.js --run 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