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

plugin/skills/using-scribe/SKILL.md

@@ -111,6 +111,12 @@ Two constraints on *how* that's achieved:
    is just a task — don't wrap it in a milestone. Either way, do not write
    plans/specs to local `.md` files. See the **writing-plans** skill.
 
+   **Find the plan before you make one.** A project's existing milestones are
+   often its roadmap, and a milestone with no steps yet is still open work.
+   `search(content_type="milestone", project_id=...)` finds one by purpose. When
+   an active milestone covers the work, add steps to it rather than opening
+   another, and give any task you record for that work its `milestone_id`.
+
 5. **Keep state honest.** Set a task `in_progress` when you start it, `done` the
    moment it's complete; log progress as you go. Always log when you
    **complete** a task and when you **hit or discover a problem**, so a change