feat(gateway): developer i_will_work_on takes a substantive step checklist (#172)

The dev plan was a free string with only a presence gate, so the
executing dev had no checklist for plan-driven progress (#173).

- IWillWorkOnRequest gains `steps` (same SubTask shape as a PM's
  sub_tasks); flow_dev route threads it through.
- i_will_work_on layers steps onto the narrative plan via the same
  panel-shaped path PMs use, so task.plan.sub_tasks is populated
  (panel render + #173 progress).
- New _dev_steps_gate (mirrors _pm_sub_tasks_gate, runs after the spec
  gate): a developer FRESH claim must supply a non-empty steps list
  with every description >= _PM_SUBTASK_DESC_MIN_LEN. Re-entry/recovery
  short-circuit before the gate (extracted _dev_reentry +
  _fresh_dev_claim keep i_will_work_on within the
  return-count + cyclomatic gates).
- developer role prompt: steps template + "thin steps rejected" + the
  progress(plan_step=...) handoff.
- Updated every dev-fresh-claim test fixture across the suite to pass
  substantive steps; added dedicated _dev_steps_gate coverage.

Commit 2 of 3 for the plan/progress quality work (#171/#172/#173).

agents/prompts/roles/developer.md

@@ -18,7 +18,7 @@ You write code; you do not coordinate. If you find yourself thinking "let me als
 | Verb | What it does | Preconditions |
 |---|---|---|
 | `give_me_work()` | Returns your highest-priority task or `idle`. | None. |
-| `i_will_work_on(task_id, plan)` | Claims a `pending`/`needs_revision` task; resumes a `claimed`/`in_progress` task you own. Auto-creates branch on first claim. **`plan` is REQUIRED** — even on first claim. The gateway returns `tracing_gap missing=['plan']` if you call without it. On resume, pass `plan='resume: <next step>'`. | Task assigned to you (or unassigned and matches your role/team); journal `decision` recorded; non-empty `plan`. |
+| `i_will_work_on(task_id, plan, steps)` | Claims a `pending`/`needs_revision` task; resumes a `claimed`/`in_progress` task you own. Auto-creates branch on first claim. **`plan` is REQUIRED** (narrative; `tracing_gap missing=['plan']` if absent). **On a FRESH claim `steps` is REQUIRED and gated** — a non-empty list of `{title, description}` where every `description` is **≥60 chars** saying what that step actually does. `steps` is your execution checklist AND your progress checklist: as you finish each, call `progress(task_id, plan_step=<step id/order>, message=...)` and the % is computed from the checklist for you. Thin/title-only steps are rejected. Example step: `{"title": "Edit README", "description": "prepend the smoke-test HTML comment above the H1, leaving the rest of the file untouched"}`. **On resume** (`claimed`/`in_progress` you own) pass `plan='resume: <next step>'`; steps are not re-required. | Task assigned to you (or unassigned and matches your role/team); journal `decision` recorded; non-empty `plan`; substantive `steps` on fresh claim. |
 | `commit(message)` | Makes the git commit, auto-prefixes `[task-id]`, records a progress entry. This is the ONLY way to commit — the gateway covers the actual git operation. | Task in `in_progress`; on your branch. |
 | `open_pr(task_id)` | Push your branch and open a PR. Run after your last commit, before `i_am_done`. `open_pr` is the finish line for *creating* the PR; use `pr_update` if you need to edit metadata afterward. | Task assigned to you; at least one commit; no PR yet. |
 | `pr_update(task_id, title?, body?, reviewers?)` | Update an existing PR's title, body, or reviewer list. Use after `open_pr` if you need to correct title/body or assign a reviewer. At least one field must be set. **Do NOT bash-shim `gh pr edit`** — that path is blocked; this verb is the gateway-native replacement. | Task has `pr_number`; you are the assignee (or your PM). |