feat(planning): start_planning hands back the active plan that already covers the work (#4079)

Step 4 of milestone 415 "An existing plan is found before a new one is made".
A session that could not see an existing plan made a second one beside it.
start_planning and create_milestone now ask first: an ACTIVE milestone in the
project with the same title, or one that reads as the same plan (title, design
and steps against milestone embeddings), is returned with its progress and a
pointer to create_records(milestone_id=...). Nothing is created; force=true
bypasses.

- dedup.find_matching_plan / plan_gate / plan_match_response; access-checked
  before either arm (rule 78), fail-open like the other gates.
- Done milestones never block; the semantic arm needs 200+ chars of candidate.
- kb_plan_match_threshold (default 0.90) is a setting, in the Settings view,
  and pinned against the Python default by test_settings_defaults_agree.

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

src/scribe/mcp/tools/tasks.py

@@ -531,17 +531,33 @@ async def start_planning(
         steps: The plan's step-tasks, in order — each an object with `title`
             (required) and optionally `body`, `status`, `priority`, `kind`
             ('work' | 'issue' | 'spike'), `tags`, `system_ids`.
-        force: Bypass the near-duplicate gate on the steps. By default a step
-            that near-duplicates an existing task blocks the whole plan, and
-            nothing — milestone included — is created.
+        force: Bypass both duplicate gates. By default nothing is created —
+            milestone included — when an ACTIVE plan in the project already
+            has this title or reads as the same plan, or when a step
+            near-duplicates an existing task. Pass it once you have read the
+            match and know this is separate work.
 
     Returns the milestone, the project's applicable rules and brief context,
     plus `steps` (the created tasks, in order) when steps were given — OR a
-    duplicate payload naming the `record` that matched, with nothing created.
+    duplicate payload with nothing created: `existing_milestone` when a plan
+    already covers this (add your steps to it with create_records(
+    milestone_id=...)), or `record` naming the step that matched a task.
     """
     uid = current_user_id()
     items = _batch_items(steps or [], what="step")
     await refuse_guessed_ids(body, *[t for item in items for t in (item.title, item.body)])
+    if not force:
+        # The plan before its steps: when a plan already covers this, the
+        # answer is to add these steps THERE, and a step-level match would
+        # only name one symptom of that.
+        match = await dedup_svc.plan_gate(
+            uid, project_id, title,
+            dedup_svc.plan_candidate_text(
+                body=body, step_texts=[f"{i.title}\n{i.body or ''}" for i in items],
+            ),
+        )
+        if match is not None:
+            return match
     if items and not force:
         dup = await _first_duplicate(uid, items, project_id or None)
         if dup is not None: