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>

frontend/src/views/TaskEditorView.vue

@@ -579,8 +579,10 @@ useEditorGuards(dirty, save);
             <label class="sb-label">Kind</label>
             <select v-model="kind" @change="markDirty" class="sb-select">
               <option value="work">Work</option>
-              <option value="plan">Plan</option>
               <option value="issue">Issue</option>
+              <!-- 'plan' is retired (plans are milestones via start_planning);
+                   offered only so legacy plan-tasks display their kind. -->
+              <option v-if="kind === 'plan'" value="plan">Plan (legacy)</option>
             </select>
           </div>
           <div v-if="startedAt || completedAt" class="sb-timestamps">