launcher._try_advance_stage and plane._try_advance_stage are now thin
wrappers over stage_engine.advance_stage. The plane webhook calls the sync
engine via asyncio.to_thread so there is exactly one implementation. The
launcher forwards finished_agent so the agent-specific rollback branches still
fire; the webhook passes None (human :approved:), matching prior behavior.
Also fixes the agent-selection bug in the launcher path: it used to enqueue
get_agent_for_stage(next_stage) (skipping a stage, e.g. analysis->architecture
launched developer instead of architect). The unified engine uses
get_agent_for_stage(current_stage), consistent with plane and gitea.
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