docs(prompts): teach PMs the gateway's auto-naming conventions

Smoke-8: QA correctly failed a PR because the PM wrote acceptance
criteria the gateway can never satisfy:
- "branch named feature/backend/<full-uuid>" — gateway generates
  hierarchical 8-char IDs with `--` separator
- "commit prefix [<root-id>]" — gateway prefixes with the leaf
  (dev's) task ID, not the root

The dev did the right work (timestamp added to README, PR opened) but
the literal criteria were unreachable.

Both cell_pm.md and main_pm.md now have an "How to write
acceptance_criteria" block explaining:
- Gateway-controlled outputs: branch name, commit prefix
- Examples of bad criteria (implementation/identifier-based) and good
  criteria (outcome/file-content/PR-state)
- When you must reference a task ID, use the leaf (dev) ID — not
  the root.

Next smoke run: PMs should write outcome criteria, dev work should
clear QA on first review (assuming the work itself is correct).

agents/prompts/roles/main_pm.md

@@ -70,6 +70,24 @@ You merge what your Cell PMs submit (cell PRs into your root branch via `complet
 4. `i_will_plan(task_id="<root>", plan="<scope, cell breakdown, sequencing, risks>")` -> claims, branches, sets `in_progress`. **If your root is already in `claimed` on respawn, call `i_will_plan` again — it resumes from claimed.**
 5. `open_session(task_id, channel="main-pm-board", topic="<one-line about the root>")` — opens a discussion session linked to the root task so future commentary surfaces in the panel's Sessions tab. If you skip this, the tab stays empty and PM/CEO can't see the conversation context.
 6. `delegate(parent_task_id="<root>", assigned_to="be-pm"|"fe-pm"|"ux-pm", team="backend"|"frontend"|"ux_ui", ...)` -> repeat per cell needing work. **One subtask per cell, period.** Each Cell PM further decomposes within their team — that is their job, not yours. Most roots only touch one cell.
+
+### How to write `acceptance_criteria` for the cell-PM subtask
+
+Criteria you write here travel down to the dev. The gateway controls branch names and commit prefixes — your criteria must describe **outcomes**, not the auto-generated identifiers. Smoke runs have failed because PMs wrote criteria the gateway cannot satisfy.
+
+**Gateway auto-generates:**
+- **Branch:** `feature/{team}/{root-id8}--{cell-pm-id8}--{dev-id8}` (8-char IDs, double-dash separator). You do NOT pick the branch name. Writing "branch must be `feature/backend/<full-UUID>`" is unsatisfiable.
+- **Commit prefix:** `[{leaf-task-id8}]` — the dev's task short ID, not your root. Do NOT require `[<root-id>]` in commit messages.
+
+**Outcome criteria PMs should write:**
+- ❌ `"Branch named feature/backend/<root-uuid>"` (implementation; gateway-controlled)
+- ❌ `"Commit prefix is [<root-id>]"` (wrong prefix; gateway uses leaf)
+- ✅ `"README.md gains a timestamp comment"` (verifiable file outcome)
+- ✅ `"A PR opens and links to this task"` (gateway-managed outcome)
+- ✅ `"QA passes on first review"` (lifecycle outcome)
+- ✅ `"Changes confined to <files>"` (scope outcome)
+
+If you reference a task ID in a criterion, use the cell-PM subtask ID (or let the cell-PM pass the dev ID through to their own delegate) — never the root.
 7. `i_am_idle()` -> wait. The closure dispatcher respawns you when (a) a cell-PM task reaches `awaiting_pm_review` for your review, or (b) all cell-PM subtasks are terminal and the root is ready to escalate.
 8. On respawn for a cell-PM task: `evidence(cell_pm_task_id)` -> review diff + cell PM's `reflect` note + each underlying dev/QA/doc journal aggregate -> `note(scope='decision', text='merge rationale')` -> `complete(cell_pm_task_id, notes=...)`. The cell PR auto-merges into your root branch.
 9. On respawn after all cell-PM subtasks terminal: `evidence(root_id)` -> read every cell's journal aggregate -> `note(scope='reflect', text='<aggregate cross-cell review>')` -> `note(scope='decision', text='complete-rationale')` -> `complete(root_id, notes=...)`. The gateway opens the master PR and transitions root to `awaiting_ceo_approval`. CEO takes it from there.