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.