fix(gateway): cell PM completes its own cell task; drop main-PM handoff

submit_up bubbled the cell task to Main PM (_handoff_to_main_pm), but
main_pm_complete rejects any task with a parent_task_id ("only operates
on root tasks"), so the cell->root PR had no one to merge it and the
cell task wedged at awaiting_pm_review. _maybe_advance_parent_to_pm_review
already intends the CELL PM to complete it.

Cell PM now owns cell completion:
- submit_up no longer hands off to Main PM; the cell task stays assigned
  to the cell PM, which is respawned to complete() it. Removed the
  now-unused _handoff_to_main_pm.
- cell_pm_complete resolves the merge target from the parent task's real
  branch_name (shared merge_chain.resolve_parent_branch, also used by the
  PR side-effects) so the cell->root PR merges into feature/main_pm/...,
  not the team-mis-derived feature/<cellteam>/... (same root cause as the
  prior PR-base fix).
- submit_up description + next_hint updated; lifecycle artifacts regen.

Main PM still only completes the ROOT (root->master + escalate-to-CEO).
First run to reach cell-PM bubble-up exposed this.

docs/rag/lifecycle/intent-verbs.md

@@ -167,7 +167,7 @@ Resume a paused task you own. paused -> in_progress.
 
 ## submit_up
 
-Cell PM bubbles a finished cell-scope task up to Main PM.
+Cell PM opens the cell→root PR and moves the cell task to awaiting_pm_review. The same Cell PM then completes it.
 
 **Allowed roles:** cell_pm