feat(adapter): Phase 6 Slice C scaffold — ItemWriter interface + Server.Writes + MBrianWriter stub

Mechanism-independent groundwork for the write-path migration. Mirrors the
slice-B ItemReader pattern:

- store/adapter.go: ItemWriter interface extracted from *Store's write
  surface (Create/Update/Reparent/AddParent/SetPublic/SetPinned/
  SoftDelete/SoftDeleteCascade + AddLink/AddLinkDated/DeleteLink), with a
  compile-time witness that *Store satisfies it.
- store/mbrian_writer.go: MBrianWriter stub — an HTTP client (NOT a pgx
  writer) against mBrian's scoped /api/projax/* write API per head's
  mechanism call (option c). Bodies return errNotImplementedSliceC until
  the HTTP impl + final spec land. Holds a pool for write-path read-backs.
- web/server.go: Server.Writes field (twin of Items), defaulted to the
  concrete *Store in web.New. main.go will flip Items+Writes atomically —
  the slice-B half-flip (reads mBrian, writes projax.items) was the bug.

No behaviour change: both Items and Writes still resolve to *Store.

web/server.go

@@ -80,8 +80,15 @@ type Server struct {
 	// aggregator depends on. Phase 6 Slice B introduces it: today the
 	// concrete *Store satisfies the ItemReader interface (legacy path);
 	// after the mBrian backend rollout PROJAX_BACKEND=mbrian wires
-	// *store.MBrianReader here. Writes still flow through Store.
+	// *store.MBrianReader here.
 	Items       store.ItemReader
+	// Writes is the write-path adapter every UI write handler, the
+	// /admin/bulk apply path, and the MCP write tools depend on. Phase 6
+	// Slice C introduces it as the twin of Items: today the concrete
+	// *Store satisfies ItemWriter (legacy path); PROJAX_BACKEND=mbrian
+	// wires *store.MBrianWriter here. main.go flips Items + Writes
+	// together — never one without the other (the slice-B half-flip bug).
+	Writes      store.ItemWriter
 	pages       map[string]*template.Template
 	Logger      *slog.Logger
 	Auth        *AuthConfig // nil → no auth (local dev / tests)
@@ -374,9 +381,11 @@ func New(s *store.Store, logger *slog.Logger) (*Server, error) {
 
 	return &Server{
 		Store: s,
-		// Default Items satisfier is *Store itself. main.go can override
-		// post-construction (e.g. PROJAX_BACKEND=mbrian → MBrianReader).
+		// Default Items + Writes satisfier is *Store itself. main.go can
+		// override both post-construction, atomically (PROJAX_BACKEND=mbrian
+		// → MBrianReader + MBrianWriter).
 		Items:       s,
+		Writes:      s,
 		pages:       pages,
 		Logger:      logger,
 		dashboard:   cache.NewTTL[*dashboardPayload](dashboardCacheTTL),