feat(roles): expose pr_update to dev/doc/cell_pm/main_pm + prompt updates

Adds pr_update to _DEV_DO, _DOC_DO, _CELL_PM_DO, _MAIN_PM_DO so the
spawn manifest builder registers it on those roles' do-servers. QA,
auditor, and Board roles do not get it — QA reviews PRs but does not
edit them; Board operates above the PR layer; auditor is silent.

developer.md and documenter.md grow a verb-table row and a note next
to the open_pr workflow step calling out that pr_update — not bash-
shimmed `gh pr edit` — is the way to fix PR metadata.

agents/prompts/roles/documenter.md

@@ -20,6 +20,7 @@ You do NOT re-implement the developer's work. You do NOT review or critique the
 | `give_me_work()` | Returns a task in `awaiting_documentation` or `idle`. | None. |
 | `claim_doc_task(task_id)` | Claims the doc task; returns PR data inline. | Task in `awaiting_documentation`; you are not the original developer. |
 | `commit(message)` | Commits doc changes on the task branch (auto-prefixed `[task-id]`). | Task in `in_progress`; on the task branch. |
+| `pr_update(task_id, title?, body?, reviewers?)` | Update the PR's title, body, or reviewer list (e.g. to add a doc-relevant summary). At least one field must be set. **Do NOT bash-shim `gh pr edit`** — use this verb. | Task has `pr_number`; you are claimant on the doc task. |
 | `i_documented(task_id, notes, files)` | Marks docs complete; transitions toward `awaiting_pm_review`. | At least one doc file in `files`; `notes` >= 20 chars. |
 | `unclaim(task_id)` | Release this claim back to pending. Use sparingly — your work-in-progress branch survives but the task is unassigned. | Task assigned to you and in claimed/in_progress. |
 | `resume(task_id)` | Resume a paused task. Transitions paused → in_progress. | Task assigned to you and in paused state. |