refactor(routes): one supersession seam for REST and MCP; PUT/PATCH notes share a handler; shared mask/not-found/caller helpers (#2829, milestone 296 area 5)

Reading the 28 route modules against each other and against the MCP tools:
- routes/notes.py carried a PUT and a PATCH handler that were the same
  function minus the supersedes contract on one of them — one handler now
  serves both verbs, so both carry it.
- The two _attach_supersession copies (REST + MCP) become
  supersession_svc.attach_relations(uid, note_id, data, hint=) — the seam
  the two surfaces must agree through; only the agent surface adds the
  one-sentence reading hint.
- Three local _uid() wrappers over g.user.id → scribe.auth.get_current_user_id
  like every other module; design_systems' private _not_found → routes.utils.
  not_found; the four "********" literals → settings_svc.SECRET_MASK with the
  read/write contract written once.
- routes/plugin.py: the project_id/repo resolution block and the
  comma-separated id parse were copied into three endpoints — _project_scope()
  and _int_list() now.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/routes/notes.py

@@ -26,22 +26,6 @@ from scribe.services import dedup as dedup_svc
 from scribe.services import supersession as supersession_svc
 from scribe.services.note_usage import record_pulled
 
-
-async def _attach_supersession(uid: int, note_id: int, data: dict) -> None:
-    """Both directions of the supersession relation on a note payload.
-
-    Mirrors the MCP helper of the same name — the two surfaces must agree about
-    what a note's payload says, or the web UI and the agent would disagree about
-    whether a record is current.
-
-    Omitted when empty: a field that always says nothing trains readers to skip
-    fields, which is what `consolidated_at` cost (#2483).
-    """
-    rel = await supersession_svc.get_relations(uid, note_id)
-    if rel["supersedes"]:
-        data["supersedes"] = rel["supersedes"]
-    if rel["superseded_by"]:
-        data["superseded_by"] = rel["superseded_by"]
 from scribe.services.note_versions import list_versions, get_version
 
 logger = logging.getLogger(__name__)
@@ -142,7 +126,7 @@ async def create_note_route():
             # may not write the target. The note itself was created.
             return jsonify({"error": str(exc), "note": note.to_dict()}), 403
     out = note.to_dict()
-    await _attach_supersession(uid, note.id, out)
+    await supersession_svc.attach_relations(uid, note.id, out)
     return jsonify(out), 201
 
 
@@ -241,13 +225,17 @@ async def get_note_route(note_id: int):
     # injected line useful?" is answered by agent pulls alone, and a human
     # clicking a link would inflate exactly the number #1038 and #2085 gate on.
     record_pulled(user_id=uid, note_id=note_id, source="rest_note")
-    await _attach_supersession(uid, note_id, data)
+    await supersession_svc.attach_relations(uid, note_id, data)
     return jsonify(data)
 
 
-@notes_bp.route("/<int:note_id>", methods=["PUT"])
+@notes_bp.route("/<int:note_id>", methods=["PUT", "PATCH"])
 @login_required
 async def update_note_route(note_id: int):
+    """Partial update — only the keys present in the payload change. PUT and
+    PATCH are the same handler on purpose: the form sends the field set it
+    edited, and the two verbs used to be two near-identical copies of this
+    function that drifted (one carried the supersedes contract, one did not)."""
     uid = get_current_user_id()
     # Share-aware: resolve through the ACL and write as the OWNER, so a shared
     # editor's save isn't rejected by the owner-scoped update service.
@@ -290,44 +278,10 @@ async def update_note_route(note_id: int):
         except PermissionError as exc:
             return jsonify({"error": str(exc)}), 403
     out = note.to_dict()
-    await _attach_supersession(uid, note_id, out)
+    await supersession_svc.attach_relations(uid, note_id, out)
     return jsonify(out)
 
 
-@notes_bp.route("/<int:note_id>", methods=["PATCH"])
-@login_required
-async def patch_note_route(note_id: int):
-    uid = get_current_user_id()
-    result = await get_note_for_user(uid, note_id)
-    if result is None:
-        return not_found("Note")
-    note_obj, _ = result
-    if not await can_write_note(uid, note_id):
-        return jsonify({"error": "Permission denied"}), 403
-    owner_uid = note_obj.user_id
-    data = await request.get_json()
-    fields = {}
-    for key in ("title", "body", "description", "parent_id", "project_id", "milestone_id", "status", "priority", "note_type"):
-        if key in data:
-            fields[key] = data[key]
-    if "due_date" in data:
-        if data["due_date"]:
-            result = parse_iso_date(data["due_date"], "due_date")
-            if isinstance(result, tuple):
-                return result
-            fields["due_date"] = result
-        else:
-            fields["due_date"] = None
-    if "tags" in data:
-        fields["tags"] = data["tags"]
-    try:
-        note = await update_note(owner_uid, note_id, **fields)
-    except ValueError as e:
-        return jsonify({"error": str(e)}), 400
-    if note is None:
-        return not_found("Note")
-    return jsonify(note.to_dict())
-
 
 @notes_bp.route("/<int:note_id>", methods=["DELETE"])
 @login_required