18 tests: happy-path advance per stage with correct agent (ORCH-4 fix),
QG-fail no-advance, reviewer REQUEST_CHANGES rollback+retry/alert, tester FAIL
rollback+retry/block, architect conflict rollback to analysis, analyst
approved-flow no-advance, and launcher+plane both delegating to the engine.
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.
Merge the two diverged _try_advance_stage implementations (launcher sync +
plane async) into one synchronous engine. Preserves all launcher business
logic (analyst approved-flow, reviewer REQUEST_CHANGES rollback+retry, tester
FAIL rollback+retry, architect conflict rollback) and the plane
check_review_approved PR-by-branch dispatch. Unifies the QG signature
dispatch. Fixes agent selection: advancing FROM current_stage launches
get_agent_for_stage(current_stage), not next_stage.
