Structure kernel terminate stage with event delivery guarantee

Refactor the termination phase (stop_container, cleanup_scratch, release_resources) into a TerminateStageHandler under agent/stage/. RETURNS A RESPONSE ONLY - no event emission. Local phase transitions: -> TERMINATING, TERMINATING -> TERMINATED. Teardown must be idempotent and leave no orphan container.

The Agent does NOT emit a CANCELLED transition - CANCELLED/reschedule belongs to sokovan session scheduling.

Do NOT emit KernelTerminating/TerminatedAnycastEvent from the handler - transitions are reported by the transition-event layer (BA-6851).

Success Criteria:
- [ ] Termination runs through TerminateStageHandler
- [ ] Handler returns a response only (no event emission)
- [ ] No orphan container remains on termination failure (retry or fallback cleanup)
- [ ] pants test passes for affected packages

JIRA Issue: BA-6079