Structure kernel create stage with event delivery guarantee

Refactor the create phase of create_kernel() into a CreateStageHandler under agent/stage/kernel_lifecycle/docker/. The handler runs create_container -> start_container and RETURNS A RESPONSE ONLY - it does not emit lifecycle events. Local phase transitions: -> CREATING, CREATING -> RUNNING.

The handler calls BEP-1057 ComputeBackend for instance creation. It must be idempotent: detect an existing instance via the kernel-id correlation key and avoid duplicate creation.

Do NOT emit KernelCreatingAnycastEvent from the handler - status transitions are reported by the separate transition-event layer (BA-6851).

Success Criteria:
- [ ] Container creation runs through CreateStageHandler, not inline in create_kernel()
- [ ] Handler returns a response only (no event emission)
- [ ] Re-run detects an existing instance and does not double-create
- [ ] pants test passes for affected packages

JIRA Issue: BA-6077