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/milestones.py

@@ -12,6 +12,7 @@ Sentinels:
 from __future__ import annotations
 
 from scribe.mcp._context import current_user_id
+from scribe.services import dedup as dedup_svc
 from scribe.services import milestones as milestones_svc
 from scribe.services import notes as notes_svc
 from scribe.services import rulebooks as rulebooks_svc
@@ -71,6 +72,7 @@ async def create_milestone(
     description: str = "",
     body: str = "",
     status: str = "active",
+    force: bool = False,
 ) -> dict:
     """Create a milestone within a Scribe project.
 
@@ -85,9 +87,21 @@ async def create_milestone(
         description: Optional one-line summary of what this milestone covers.
         body: Optional plan/design (markdown) — the milestone's full plan text.
         status: active (default) or done.
+        force: Bypass the plan gate. By default an active milestone that
+            already has this title, or reads as the same plan, is returned
+            (`existing_milestone`) and nothing is created.
     """
     uid = current_user_id()
     await refuse_guessed_ids(title, description, body)
+    # A done milestone is a record of past work, not a competing plan, so
+    # only an active create is gated — the same line find_matching_plan draws.
+    if status == "active" and not force:
+        match = await dedup_svc.plan_gate(
+            uid, project_id, title,
+            dedup_svc.plan_candidate_text(description=description, body=body),
+        )
+        if match is not None:
+            return match
     milestone = await milestones_svc.create_milestone(
         uid,
         project_id=project_id,