fix(stop-continuation): persist stop state across user messages (#3276)

The /stop-continuation command was ineffective because the stop-
continuation-guard cleared its stopped state on the very next
chat.message event. Since any user message (including normal chat
after stopping) triggers chat.message, the continuation would
resume immediately.

Root cause: the chat.message handler called clear(sessionID) on
every user message, treating it as a 'user resumed work' signal.
But the user expects /stop-continuation to persist until they
explicitly start work again.

Changes:
- stop-continuation-guard chat.message: no longer clears stop state
- tool-execute-before: /start-work, /ralph-loop, /ulw-loop now
  explicitly clear the stop state (so continuation resumes when
  user intentionally restarts work)
- Updated and added tests: 12 pass (3 new), 125 related tests pass

Closes #3276