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/mcp/tools/notes.py

@@ -62,30 +62,6 @@ async def list_notes(
     return {"notes": [n.to_dict() for n in rows], "total": total}
 
 
-async def _attach_supersession(uid: int, note_id: int, data: dict) -> None:
-    """Add both directions of the supersession relation to a note payload.
-
-    Both, because they answer different questions and only one of them is
-    obvious. `supersedes` is what the author claimed. `superseded_by` is what a
-    READER needs and what the note itself cannot know — a stale record handed
-    over without that marker gets acted on confidently, which is worse than
-    never surfacing it.
-
-    Omitted entirely when empty, so an ordinary note's payload doesn't grow two
-    permanently-empty lists. A field that always says nothing trains readers to
-    skip fields, which is the lesson `consolidated_at` cost us (#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"]
-        data["superseded_note"] = (
-            "A later note claims to bring this up to date — see superseded_by. "
-            "Read this as what was true when written, and check the newer one "
-            "before acting on it."
-        )
-
 
 async def get_note(note_id: int) -> dict:
     """Fetch the full content of a single Scribe note by its ID.
@@ -113,7 +89,7 @@ async def get_note(note_id: int) -> dict:
     # snippets would leave those permanently at zero pulls and make them look
     # like dead weight next to snippets that merely had a counter (#2085).
     record_pulled(user_id=uid, note_id=int(note.id), source="mcp_get_note")
-    await _attach_supersession(uid, note_id, out)
+    await supersession_svc.attach_relations(uid, note_id, out, hint=True)
     await systems_tools.attach_systems(
         uid, getattr(note, "user_id", uid) or uid, out, note.id, note.project_id
     )
@@ -186,7 +162,7 @@ async def create_note(
             raise ValueError(str(exc)) from exc
     data = note.to_dict()
     await systems_tools.attach_systems(uid, uid, data, note.id, project_id or None)
-    await _attach_supersession(uid, note.id, data)
+    await supersession_svc.attach_relations(uid, note.id, data, hint=True)
     return data
 
 
@@ -237,7 +213,7 @@ async def update_note(
     await systems_tools.attach_systems(
         uid, getattr(note, "user_id", uid) or uid, data, note_id, note.project_id
     )
-    await _attach_supersession(uid, note_id, data)
+    await supersession_svc.attach_relations(uid, note_id, data, hint=True)
     return data