fix: harden workspace registry deployment

backend/src/routes/sessions.ts

@@ -32,6 +32,8 @@ export function sessionRoutes(
     workspaceRegistry: WorkspaceRegistry;
     /** Local-only guard: probe DWH reachability before creating a session (run-stack.sh). */
     dwhPrecheck?: boolean;
+    /** Explicit loopback-only compatibility path for old clients that send `workspace`. */
+    legacyWorkspaceMode?: boolean;
   },
 ) {
   const lifecycleTails = new Map<string, Promise<void>>();
@@ -267,9 +269,10 @@ export function sessionRoutes(
     try { s = await d.getSettings(principal); } catch { return storageFailure(reply); }
     const runner = runnerFor(principal);
     // `workspace` was the legacy request field before browser-local registry preferences.
-    // It opts a pre-registry caller into the existing installation-default config only; modern
-    // `workspaceId` and saved preferences must continue to resolve an immutable snapshot.
-    const legacyWorkspaceRequest = typeof b.workspace === "string" && b.workspace.length > 0;
+    // It is available only through the explicit loopback-only compatibility mode; every normal
+    // new session must resolve and pin an immutable registry revision.
+    const legacyWorkspaceRequest = d.legacyWorkspaceMode
+      && typeof b.workspace === "string" && b.workspace.length > 0;
     const requestedWorkspaceId = b.workspaceId ?? (legacyWorkspaceRequest ? undefined : s.workspace);
     if (!requestedWorkspaceId && !legacyWorkspaceRequest) {
       return reply.code(409).send({