docs(prompts): teach all roles the roboco-git-readonly verbs

Smoke-8: QA fell back to Bash for git inspection (git log, git branch,
git show) — bash-guard correctly blocked most of it. The
roboco-git-readonly MCP server WAS registered for every agent (per
orchestrator.py:1897) with roboco_git_status/log/diff/branches, but
no role prompt mentioned them, so agents never tried them.

Added the four verbs to the verb tables in: developer.md, documenter.md,
qa.md, cell_pm.md, main_pm.md, board.md. Each entry notes "use these,
NOT raw `Bash git ...`" so agents reach for the right tool first.

No code change — these MCP tools have existed all along. This is a
prompt visibility fix.

agents/prompts/roles/cell_pm.md

@@ -33,6 +33,7 @@ You merge what your developers submit (leaf PRs into your cell branch via `compl
 | `say(channel, text)` / `dm(recipient, text)` | Channel post / DM. **Channel slug without `#`. Valid slugs:** cell channels (`backend-cell`, `frontend-cell`, `uxui-cell`), cross-cell (`dev-all`, `qa-all`, `pm-all`, `doc-all`), management (`main-pm-board`, `board-private`), broadcast (`announcements`, `all-hands`). Inventing a slug ("backend-dev", "backend") returns `Channel not found`. | None. |
 | `notify(target, text, priority?)` | Send a formal ack-required notification to an agent (`be-dev-1`, `ceo`, etc.). `priority` is one of `normal`/`high`/`urgent` (default `normal`). | None. |
 | `evidence(task_id)` | Inspect a task's PR + commits + diff. | None. |
+| `roboco_git_status(project_slug)` / `roboco_git_log(project_slug, limit?, branch?)` / `roboco_git_diff(project_slug, branch?, base?)` / `roboco_git_branches(project_slug)` | Read-only git inspection. Use these (not raw `Bash git ...`) when you need to verify a subtask's branch state before completing/merging. | None. |
 | `i_am_idle()` | Exit cleanly; auto-pauses any `in_progress` tasks you own so you'll be respawned at the right moment. Soft-blocks on unread notifications — clear inbox first via `notify_list` → `notify_get` → `notify_ack`. | None. |
 | `open_session(task_id, channel, topic, relationship_type='discussion')` | Open a discussion session linked to a task — populates the panel's Sessions tab. Use when starting work on a non-trivial child task that needs a discussion thread. `channel` is a valid slug from the channel list. | Caller must be PM-or-up; task must exist. |
 | `link_session(session_id, task_id, is_primary=False)` | Link an existing session to another task (idempotent). | You must own the task. |