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/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "scribe",
   "description": "Scribe for Claude Code: connects the scribe MCP server, adds the hooks that deliver live project state and relevant records at the right moment, ships the shared client-neutral Scribe skills (using-scribe, writing-plans, reporting-back, systematic-debugging, verification, brainstorming, reusing-code, shape-accounting), and syncs your saved Scribe Processes as skills (/scribe:sync).",
-  "version": "2026.09.15.1626",
+  "version": "2026.09.15.1744",
   "author": {
     "name": "Bryan Van Deusen"
   },

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

plugin/skills/writing-plans/SKILL.md

@@ -26,6 +26,34 @@ a complete, honest record of work that didn't need a plan.
 Some projects are milestone-shaped and some are a flat task list. Read the
 project you are in rather than imposing a shape on it.
 
+## Then look for the plan that already exists
+
+Before you start a plan, find out whether the project already has one for this
+work. A roadmap is often written ahead of the work as milestones with a goal
+and no steps yet, and those are exactly the plans a session misses: they have
+nothing in them to show up as open tasks.
+
+- `enter_project` lists the recent milestones and, under `unplanned_milestones`,
+  the active ones with no steps.
+- `search(content_type="milestone", project_id=...)` finds a plan by what it is
+  for. `list_milestones` reads the whole roadmap.
+
+**When an active milestone already covers the work, the plan is that milestone.**
+Read it with `get_milestone`, add your steps with
+`create_records(milestone_id=<its id>, records=[…])`, and revise its body with
+`update_milestone` if what you know now changes the design. Opening a new
+milestone beside it splits one piece of work across two plans, and neither
+shows the whole. A second milestone is right only when the work is a separate
+arc: a different goal that would still make sense if the first plan were done.
+
+The same holds for single tasks. Work that belongs to an existing plan is
+created with that milestone's `milestone_id`, not left loose beside it.
+
+`start_planning` and `create_milestone` check too. When an active milestone in
+the project has the same title or reads as the same plan, they return it as
+`existing_milestone` and create nothing. Add to that plan, or pass `force=true`
+only when you have read it and this really is separate work.
+
 ## When it does: start the plan in Scribe, not a file
 
 Call **`start_planning(project_id, title)`** before designing or implementing —
@@ -59,7 +87,7 @@ record, not a file on disk. (The old `kind=plan` task is retired; `start_plannin
 no longer creates one.)
 
 Before designing from scratch, **recall**: `search` Scribe for a related prior
-plan or decision. Often the thinking (or half of it) already exists.
+decision or note. Often the thinking (or half of it) already exists.
 
 ## What a good plan contains