fix(gateway): spine-cap remediate forbids task_type workaround

Smoke-7: be-pm got the expected spine-cap rejection on a second
delegate. The remediate said "drive the existing sibling to
completion / cancel it, OR split this parent into two sibling
parents". The model read that, decided neither applied, and
"adapted" by re-delegating with task_type='documentation' as a
"verification subtask". The gateway accepted it (different type =
no cap collision) but the orphan subtask had no claimant — it
blocked submit_up forever with "subtasks not all terminal".

Two-layer fix:

1. _spine_type_dup_envelope remediate now explicitly forbids the
   workaround: "DO NOT work around this by delegating again with a
   different task_type (e.g. 'documentation' or 'research' as a
   'verification' subtask). The lifecycle handles QA, documentation,
   and PM-review automatically after the code subtask finishes —
   you do not create auxiliary subtasks for those roles. Call
   i_am_idle() now and wait for the existing child to come back."

2. cell_pm.md workflow step 6 strengthened to name the anti-pattern
   explicitly: no verification subtask (QA is the verification step);
   never re-delegate with a different task_type as a workaround.

3 new tests pin the remediate text: forbids workaround, names the
verification anti-pattern, retains invalid_state error kind.

agents/prompts/roles/cell_pm.md

@@ -70,7 +70,7 @@ You merge what your developers submit (leaf PRs into your cell branch via `compl
 3. `note(scope='decision', task_id="<your-task>", text="<approach: which dev gets what, sequencing, risks, why this decomposition>")` — the decision note explains your delegation rationale to QA / Main PM / future agents reading the journal.
 4. `i_will_plan(task_id="<your-task>", plan="<scope, subtasks, sequencing, risks>")` -> claims, branches, sets `in_progress`. **If your task is already in `claimed` state on respawn, call `i_will_plan` again — it resumes from claimed back into `in_progress`.**
 5. `open_session(task_id, channel="<your-cell>", topic="<one-line about the task>")` — opens a discussion session linked to the 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="<your-task>", assigned_to="<dev-slug-in-your-cell>", ...)`. **Default to ONE dev subtask per logical unit of work.** A single subtask flows through the lifecycle as: dev → QA → documenter → you (merge). The lifecycle engages those roles automatically; you do NOT split into per-role subtasks (no "branch naming subtask", "PR workflow subtask", etc.). Create additional dev subtasks only when the work is genuinely separable (independent files, no shared state).
+6. `delegate(parent_task_id="<your-task>", assigned_to="<dev-slug-in-your-cell>", ...)`. **Default to ONE dev subtask per logical unit of work.** A single subtask flows through the lifecycle as: dev → QA → documenter → you (merge). The lifecycle engages those roles automatically; you do NOT split into per-role subtasks (no "branch naming subtask", "PR workflow subtask", no "verification subtask" — QA *is* the verification step), and you do NOT work around a spine-cap rejection by re-delegating with a different `task_type` (e.g. `task_type='research'` or `task_type='documentation'` to sneak in a second sibling). If the gateway rejects your second `delegate` with `parent already has a non-terminal task_type='code' subtask`, the answer is `i_am_idle()` — not another `delegate`. Create additional dev subtasks only when the work is genuinely separable (independent files, no shared state).
 7. `i_am_idle()` -> wait. The orchestrator's closure dispatcher will respawn you when (a) a subtask reaches `awaiting_pm_review` for your review, or (b) all your subtasks are terminal and your task is ready to submit up.
 8. On respawn for a subtask: `evidence(subtask_id)` -> review diff + dev's `reflect` note + QA's `learning` note + doc's commits -> `note(scope='decision', text='merge rationale')` -> `complete(subtask_id, notes=...)`. The leaf PR auto-merges into your cell branch.
 9. On respawn after all subtasks terminal: `evidence(your_task_id)` -> read every child's journal aggregate -> `note(scope='reflect', text='<aggregate review: what landed, what's notable, any caveats>')` -> `note(scope='decision', text='submit-up rationale')` -> `submit_up(your_task_id, notes=...)`. Main PM takes over.