The keyword-detector was using ctx.client.session.get() to check parentID for
determining subagent sessions, but this API didn't reliably return parentID.
This caused non-ultrawork keywords (search, analyze) to be injected in subagent
sessions when they should only work in main sessions.
Changed to use getMainSessionID() comparison, consistent with other hooks like
session-notification and todo-continuation-enforcer.
- Replace unreliable parentID API check with mainSessionID comparison
- Add comprehensive test coverage for session filtering behavior
- Remove unnecessary session.get API call
b5c1cfb57f