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/plugin.py

@@ -24,6 +24,35 @@ plugin_bp = Blueprint("plugin", __name__, url_prefix="/api/plugin")
 _MARKETPLACE_KEY = "plugin_marketplace_url"
 
 
+def _int_list(raw: str | None) -> list[int]:
+    """A comma-separated id list from the query string; non-ints dropped."""
+    return [int(p) for p in (raw or "").split(",") if p.strip().isdigit()]
+
+
+async def _project_scope() -> tuple[int, str, str]:
+    """(project_id, repo, unbound_repo) from the request's `project_id` /
+    `repo` query args — the one resolution every plugin endpoint shares.
+
+    An explicit project_id wins; otherwise the repo remote is resolved
+    through the caller's bindings, and a remote nobody bound comes back as
+    `unbound_repo` (normalised) so /context can say "bind this repo".
+    """
+    try:
+        project_id = int(request.args.get("project_id", 0) or 0)
+    except (TypeError, ValueError):
+        project_id = 0
+    repo = (request.args.get("repo") or "").strip()
+    unbound_repo = ""
+    if repo and not project_id:
+        resolved = await repo_bindings_svc.resolve_project(g.user.id, repo)
+        if resolved:
+            project_id = resolved
+        else:
+            unbound_repo = repo_bindings_svc.normalize_repo_key(repo)
+    return project_id, repo, unbound_repo
+
+
+
 @plugin_bp.get("/context")
 @login_required
 async def session_context():
@@ -37,20 +66,7 @@ async def session_context():
         project_id (optional int) — explicit override, mainly for manual/ad-hoc
             curl testing; takes precedence over `repo` when set.
     """
-    try:
-        project_id = int(request.args.get("project_id", 0) or 0)
-    except (TypeError, ValueError):
-        project_id = 0
-
-    unbound_repo = ""
-    repo = (request.args.get("repo") or "").strip()
-    if repo and not project_id:
-        resolved = await repo_bindings_svc.resolve_project(g.user.id, repo)
-        if resolved:
-            project_id = resolved
-        else:
-            unbound_repo = repo_bindings_svc.normalize_repo_key(repo)
-
+    project_id, _repo, unbound_repo = await _project_scope()
     result = await plugin_ctx_svc.build_session_context(
         g.user.id, project_id, unbound_repo=unbound_repo
     )
@@ -77,22 +93,8 @@ async def autoinject_retrieve():
                              session; skipped so each note injects at most once.
     """
     q = (request.args.get("q") or "").strip()
-    try:
-        project_id = int(request.args.get("project_id", 0) or 0)
-    except (TypeError, ValueError):
-        project_id = 0
-
-    repo = (request.args.get("repo") or "").strip()
-    if repo and not project_id:
-        resolved = await repo_bindings_svc.resolve_project(g.user.id, repo)
-        if resolved:
-            project_id = resolved
-
-    exclude_ids = [
-        int(p) for p in (request.args.get("exclude_ids") or "").split(",")
-        if p.strip().isdigit()
-    ]
-
+    project_id, _repo, _unbound = await _project_scope()
+    exclude_ids = _int_list(request.args.get("exclude_ids"))
     result = await plugin_ctx_svc.build_autoinject_hint(
         g.user.id, q, project_id=project_id, exclude_ids=exclude_ids
     )
@@ -139,25 +141,9 @@ async def write_path_prior_art():
     """
     path = (request.args.get("path") or "").strip()
     code = request.args.get("code") or ""
-    try:
-        project_id = int(request.args.get("project_id", 0) or 0)
-    except (TypeError, ValueError):
-        project_id = 0
-
-    repo = (request.args.get("repo") or "").strip()
-    if repo and not project_id:
-        resolved = await repo_bindings_svc.resolve_project(g.user.id, repo)
-        if resolved:
-            project_id = resolved
-
-    exclude_ids = [
-        int(p) for p in (request.args.get("exclude_ids") or "").split(",")
-        if p.strip().isdigit()
-    ]
-    exclude_sync_ids = [
-        int(p) for p in (request.args.get("exclude_sync_ids") or "").split(",")
-        if p.strip().isdigit()
-    ]
+    project_id, repo, _unbound = await _project_scope()
+    exclude_ids = _int_list(request.args.get("exclude_ids"))
+    exclude_sync_ids = _int_list(request.args.get("exclude_sync_ids"))
     shapes = _parse_shapes(request.args.get("shapes") or "")
     api_key = getattr(g, "api_key", None)
     may_stamp = api_key is None or getattr(api_key, "scope", "") == "write"