feat(progress): plan-driven progress — % derived from the plan checklist (#173)

Progress was only the synthetic milestone entry (auto-emitted at
open_pr/i_am_done); agents never deliberately reported and the % was
an ungated free-form guess.

Now the plan's sub_tasks ARE the progress skeleton:
- progress() gains optional `plan_step` (a sub_task id or its 1-based
  order). With it, that step is marked completed and the percentage is
  DERIVED as completed/total (equal weight) via new
  TaskService.record_plan_progress — the agent cannot set/game it.
- A narrative entry WITHOUT plan_step is allowed for important
  mid-step documentation and carries the current derived % (the bar
  never regresses). No hard anti-spam gate (would loop minimax) —
  prompt guidance steers "meaningful moments, not every tool call".
- `percentage` is now an optional fallback, used only for tasks with
  no sub_task checklist (back-compat). v2 ProgressRequest, the do.py
  route, and the do_server MCP tool updated accordingly.
- An unmatched plan_step returns invalid_state listing the valid step
  refs (resolve by id / order / 1-based index).
- developer + documenter prompts updated to the plan_step workflow.
- Helpers extracted (_plan_subtasks/_derive_plan_pct/_valid_step_refs/
  _mark_subtask_complete) to keep record_plan_progress within the
  cyclomatic gate.

Commit 3 of 3 for the plan/progress quality work (#171/#172/#173).

agents/prompts/roles/developer.md

@@ -59,7 +59,7 @@ When you respawn, your task is in some lifecycle status. The next call follows f
 3. `note(scope='decision', text='<approach: files I'll touch, plan, risks, how I'll verify each criterion>')` -> records your reasoning before claiming.
 4. `i_will_work_on(task_id, plan="<scope, files, approach, risks>")` -> claims, creates branch, sets `in_progress`.
 5. Edit / Write your changes inside the workspace. Run tests via `Bash` after each meaningful change.
-6. `commit(message=...)` after each meaningful change. Then `progress(task_id, message="<one sentence about what just landed>", percentage=<0..100>)` to surface narrative progress to the Plan/Progress tab. Commits are git refs; progress is the human-readable update for QA / PM / CEO. Repeat 5-6 until the criteria are met.
+6. `commit(message=...)` after each meaningful change. **As you FINISH each plan step, call `progress(task_id, plan_step="<that step's id or 1-based order>", message="<one sentence about what landed>")`** — the step is marked complete and the % is computed from your checklist for you (do NOT pass `percentage`; you cannot set it). You MAY also post a `progress(task_id, message=...)` WITHOUT `plan_step` for an important mid-step milestone (documents the "why"; carries the current %) — meaningful moments only, not every tool call. Commits are git refs; progress maps to your plan for QA / PM / CEO. Repeat 5-6 until every step is done and the criteria are met.
 7. If you get stuck (test won't pass, design unclear, deps missing): `note(scope='struggle', text='<what's stuck + what you've tried>')` BEFORE moving to `i_am_blocked`. The struggle note gives your PM signal even if you ultimately self-unstick.
 8. When a struggle resolves: `note(scope='learning', text='<what worked + why>')` so the next agent benefits.
 9. `note(scope='reflect', text="<what you did + why + how each acceptance criterion was met>")` before submitting. **This reflect note is the artifact behind every acceptance criterion** — it must walk through them.

agents/prompts/roles/documenter.md

@@ -29,7 +29,7 @@ You do NOT re-implement the developer's work. You do NOT review or critique the
 | `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. |
+| `progress(task_id, message, plan_step?)` | Record progress to the panel's Progress tab. Pass `plan_step` (a sub_task id or its 1-based order) as you finish each plan step — it is marked complete and the % is **computed for you** (you do not set `percentage`). A narrative entry without `plan_step` is allowed for an important milestone. 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. |
 
 ## State → Verb