chore(plans): make kind=plan retirement consistent across MCP, REST, UI, skills

Audit of the plugin + MCP surface after milestone-as-plan (T3): every path
that could still create a kind=plan task or describe the old plan-task model
is now aligned with the hard-retire decision.

- create_task (MCP + REST POST /api/tasks): reject kind=plan with a message
  pointing to start_planning. The 'plan' enum value stays valid so legacy
  plan-tasks remain readable; update paths never touch kind, so they round-trip.
- create_task / get_task docstrings: 'plan' dropped from creatable kinds;
  get_task's rules-augmentation noted as legacy-only (get_milestone for new plans).
- skills/writing-plans: rewritten for milestone-as-plan (body = design, steps =
  child tasks, get_milestone to read back).
- skills/using-scribe: "plans live in milestones via start_planning", not kind=plan.
- TaskEditorView Kind selector: offers Work/Issue; "Plan (legacy)" shown only
  when the loaded task is already kind=plan (display round-trip).
- test: create_task rejects kind=plan.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugin/skills/using-scribe/SKILL.md

@@ -30,8 +30,8 @@ This plugin makes Scribe the home for the operator's **rules, recall, and
 planning** — the jobs Claude's native auto-memory would otherwise do. When the
 plugin is present, route those jobs to Scribe and **do not also write them to
 native memory**: codify rules with `create_rule` / `create_project_rule`,
-capture durable knowledge as Scribe notes, and keep plans in `kind=plan` tasks —
-not in `MEMORY.md` or `CLAUDE.md`. One copy, in Scribe; let any existing local
+capture durable knowledge as Scribe notes, and keep plans in Scribe milestones
+(via `start_planning`) — not in `MEMORY.md` or `CLAUDE.md`. One copy, in Scribe; let any existing local
 memory shrink as Scribe takes over. Don't maintain both stores in parallel.
 
 Two constraints on *how* that's achieved:
@@ -64,9 +64,10 @@ Two constraints on *how* that's achieved:
    note/rule/task over creating a new one. Search first; revise what's there.
 
 4. **Plans live in Scribe.** For non-trivial work call `start_planning(project_id,
-   title)` FIRST — the plan body + step checklist live in the `kind=plan` task,
-   progress goes in work-logs (`add_task_log`). Do not write plans/specs to local
-   `.md` files.
+   title)` FIRST — it creates a milestone whose `body` holds the design; each
+   step is its own task under that milestone (`create_task(milestone_id=...)`),
+   progress goes in work-logs (`add_task_log`). Read it back with `get_milestone`.
+   Do not write plans/specs to local `.md` files.
 
 5. **Keep state honest.** Set a task `in_progress` when you start it, `done` the
    moment it's complete; log progress as you go.

plugin/skills/writing-plans/SKILL.md

@@ -1,46 +1,58 @@
 ---
 name: writing-plans
-description: Use before starting any non-trivial or multi-step piece of work — produce a clear plan BEFORE diving in. Triggers when the user asks you to plan, design an approach, scope an effort, or tackle work big enough to need ordered steps. The plan lives in a Scribe kind=plan task (via start_planning), not a local file.
+description: Use before starting any non-trivial or multi-step piece of work — produce a clear plan BEFORE diving in. Triggers when the user asks you to plan, design an approach, scope an effort, or tackle work big enough to need ordered steps. The plan lives in a Scribe milestone (via start_planning), not a local file.
 ---
 
 # Writing plans
 
-A plan is **how** you'll execute a chunk of work — the design plus an ordered,
-checkable list of steps — written *before* you start, so the approach is
-reviewable and the work stays trackable.
+A plan is **how** you'll execute a chunk of work — the design plus an ordered
+set of steps — written *before* you start, so the approach is reviewable and the
+work stays trackable.
 
 ## Start the plan in Scribe, not a file
 
 For non-trivial work, call **`start_planning(project_id, title)` FIRST** —
-before any design or implementation. It creates a `kind=plan` task seeded with a
-template and returns the task id plus the project's applicable rules. The plan
-lives in that task: edit the body with `update_task`, record progress with
-`add_task_log`. **Do not** write plans or specs to local `.md` files — the task
-is the record, not a file on disk.
+before any design or implementation. It creates a **milestone** (the plan
+container) seeded with a design template and returns the milestone id plus the
+project's applicable rules. The plan lives in that milestone:
+
+- The **design/intent** goes in the milestone `body` — edit it with
+  `update_milestone(milestone_id, body=...)`.
+- Each **step** is its own task under the milestone — create it with
+  `create_task(milestone_id=<that milestone>)` and track it with status +
+  `add_task_log`. Steps are first-class tasks, **not** checkboxes in the body.
+- Read the whole plan back with `get_milestone` (body + its step-tasks).
+
+**Do not** write plans or specs to local `.md` files — the milestone is the
+record, not a file on disk. (The old `kind=plan` task is retired; `start_planning`
+no longer creates one.)
 
 Before designing from scratch, **recall**: `search` Scribe for a related prior
 plan or decision. Often the thinking (or half of it) already exists.
 
 ## What a good plan contains
 
-- **Goal** — what "done" looks like, and why, in a sentence or two.
-- **Approach** — the key design decisions and the trade-offs you chose, briefly.
-- **Steps** — an ordered checklist, each step small enough to verify on its own;
-  note which files/areas each touches.
+- **Goal** — what "done" looks like, and why, in a sentence or two (milestone body).
+- **Approach** — the key design decisions and the trade-offs you chose, briefly
+  (milestone body).
+- **Steps** — an ordered set of step-tasks under the milestone, each small enough
+  to verify on its own; note which files/areas each touches.
 - **Verification** — how you'll know it actually works (a test, CI, an
   observable behavior), not just "it's written."
 
 ## While executing
 
-- Keep the plan **honest**: tick steps as they land; record decisions, findings,
-  and pivots with `add_task_log` rather than silently rewriting the body.
-- If reality diverges from the plan, **update the plan** — one that no longer
-  matches what you're doing is worse than none.
-- Set the plan task `in_progress` when you start and `done` when it's complete.
+- Keep the plan **honest**: drive each step-task's status (todo →
+  in_progress → done) as it lands; record decisions, findings, and pivots with
+  `add_task_log` on the relevant step rather than silently rewriting the body.
+- If reality diverges from the plan, **update the milestone body** — a design
+  that no longer matches what you're doing is worse than none. Add or re-scope
+  step-tasks as the work changes.
+- Mark the milestone `done` when its steps are complete.
 
 ## Match depth to the work
 
-A two-step change deserves a two-line plan; a multi-day effort deserves
-milestones and sub-tasks. Don't over-plan the trivial, and don't under-plan
-something that will sprawl. The point is a shared, reviewable intent — not
-ceremony.
+A two-step change deserves a two-line plan; a multi-day effort deserves a
+fleshed-out milestone body and several step-tasks. Don't over-plan the trivial,
+and don't under-plan something that will sprawl. The point is a shared,
+reviewable intent — not ceremony.