Structure kernel create stage with event delivery guarantee

[HyeockJinKim]

Refactor the create phase of create_kernel() into a dedicated Handler under agent/stage/kernel_lifecycle/docker/.

Scope:

• Extract container creation logic (container prepared → container started → kernel object initialized) into a CreateStageHandler
• Guarantee that KernelCreatingAnycastEvent is emitted at stage entry and that no silent failure path bypasses the event
• On failure, emit an appropriate error event so the Manager is never left waiting
• Target files: agent/stage/kernel_lifecycle/docker/bootstrap.py, kernel_object.py, agent/agent.py

Success Criteria:

• Container creation logic runs through CreateStageHandler, not inline in create_kernel()
• KernelCreatingAnycastEvent is always emitted at stage start
• Failure path emits error event; no silent drop
• Existing create_kernel() behavior unchanged end-to-end
• pants test passes for affected packages

JIRA Issue: BA-6077