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/documenter.md

@@ -27,6 +27,7 @@ You do NOT re-implement the developer's work. You do NOT review or critique the
 | `note(text, scope?)` | Journal entry. | None. |
 | `say(channel, text)` / `dm(recipient, text, skill?)` | Channel post / direct message. | Channel slug without `#`. |
 | `evidence(task_id)` | Re-fetches PR diff and commits if needed. | 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 — verify dev's commits before drafting docs. | None. |
 | `i_am_idle()` | Done for now. Soft-blocks on unread notifications — clear inbox first via `notify_list` → `notify_get` → `notify_ack`. | No active doc claim. |
 | `progress(task_id, message, percentage)` | Append a narrative progress entry to the panel's Progress tab (0..100). Use in addition to `commit()`. **NOT `TodoWrite`** — TodoWrite is your private session scratchpad that does NOT surface to the panel. | Task assigned to you and active. |
 | `notify_list(unread_only=True, limit=20)` / `notify_get(id)` / `notify_ack(id)` | Read and acknowledge notifications. | None. |