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

tests/test_guidance_ownership.py

@@ -147,6 +147,12 @@ TOPICS: tuple[Topic, ...] = (
     # ── process arcs — owned by their skills ──
     Topic("plan in a milestone, steps created together", "skill:writing-plans", ("start_planning", "{{ref:"),
           "a milestone earns its place when the work has an arc", index=("start_planning",)),
+    # Milestone 415: sessions opened a second plan beside the roadmap milestone
+    # that already covered the work, because nothing told them to look.
+    Topic("find the existing plan before making one", "skill:writing-plans",
+          ('content_type="milestone"', "unplanned_milestones", "existing_milestone"),
+          "when an active milestone already covers the work, the plan is that milestone",
+          index=('search(content_type="milestone")',)),
     Topic("reuse recorded shapes; record at first build", "skill:reusing-code",
           ("create_snippet", "when_to_use", "first build", "second copy"),
           "prior art offered beside a write is not noise", index=("create_snippet",)),