feat(KB-336): rename data directory from .kb to .fusion

- Rename data directory from .kb to .fusion across core, CLI, dashboard, and engine
- Update all path references in source files, tests, and CLI output
- Remove deprecated central-core infrastructure (central-core, central-db, central-integration)
- Remove obsolete test files (PlanningModeModal.test.tsx, ScheduleForm.test.tsx)
- Add changeset for the directory rename breaking change

packages/core/src/agent-store.ts

@@ -84,7 +84,7 @@ export class AgentStore extends EventEmitter {
 
   constructor(options: AgentStoreOptions = {}) {
     super();
-    this.rootDir = options.rootDir ?? ".kb";
+    this.rootDir = options.rootDir ?? ".fusion";
     this.agentsDir = join(this.rootDir, "agents");
   }