docs(plugin): find the existing plan before making one, and file related work into it (#4080)

Step 5 of milestone 415 "An existing plan is found before a new one is made".
Sessions opened a second milestone beside the roadmap milestone that already
covered the work, and filed related tasks loose, because no surface told them
to look first.

- writing-plans: a section on finding the plan that exists (enter_project's
  unplanned_milestones, search(content_type="milestone"), list_milestones);
  when an active milestone covers the work, add steps to it; a second
  milestone only for a separate arc; the gate's existing_milestone reply.
- using-scribe: "when you plan" gains the same check and milestone_id on
  related tasks.
- _INSTRUCTIONS PLAN line points at the milestone search (1,689 of 2,000).
- test_guidance_ownership pins the topic on writing-plans.
- Plugin version minted: 2026.09.15.1744.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/mcp/server.py

@@ -49,8 +49,9 @@ client reads Agent Skills) and in each tool's description. The index:
 - RECALL: search before acting, scoped with the active project_id.
 - RECORD: create_task; a fix is kind="issue". add_task_log as you go; status
   in_progress on start, done on finish. Tag system_ids as you write.
-- PLAN work with an arc: start_planning(steps=[...]). The plan is a milestone
-  and each step a task.
+- PLAN work with an arc: find the existing plan first
+  (search(content_type="milestone")) and add steps to it; else
+  start_planning(steps=[...]). The plan is a milestone and each step a task.
 - IDS exist only once a create returns them. Records that cite each other go
   through create_records, writing {{ref:N}} for the Nth record.
 - REUSE: search snippets before building; create_snippet what you build.