refactor(FN-1056): remove dead code and unused CSS

- Delete SetupWizard component and its test file (no longer used)
- Remove migration-stubs.ts (dead migration helper code)
- Strip unused CSS rules from dashboard styles.css
- Fix minor test assertion in activity-feed theme test
- Add changeset for patch release

packages/core/src/migration-stubs.ts

@@ -1,27 +0,0 @@
-export interface CentralCoreStub {
-  registerProject(input: {
-    name: string;
-    path: string;
-    isolationMode?: "in-process" | "child-process";
-  }): Promise<{ id: string; name: string; path: string }>;
-  /** Compatibility overload matching the PROMPT.md fallback contract. */
-  registerProjectLegacy?(
-    name: string,
-    workingDir: string,
-    options?: { isolationMode?: string }
-  ): Promise<{ id: string; name: string; workingDirectory: string }>;
-  listProjects(): Promise<Array<{ id: string; name: string; path: string; status?: string }>>;
-  getProject(id: string): Promise<{ id: string; name: string; path: string; status?: string } | undefined>;
-  getProjectByPath(path: string): Promise<{ id: string; name: string; path: string; status?: string } | undefined>;
-  isProjectRegistered?(workingDir: string): Promise<boolean> | boolean;
-  updateProject?(id: string, updates: { status?: "active" | "paused" | "errored" | "initializing" }): Promise<{ id: string; name: string; path: string; status?: string }>;
-  getGlobalDir(): string;
-}
-
-export interface ProjectInfoStub {
-  id: string;
-  name: string;
-  workingDirectory: string;
-  status: "active" | "paused" | "errored";
-  isolationMode: "in-process" | "child-process";
-}