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/db-migrate.test.ts

@@ -16,7 +16,7 @@ describe("detectLegacyData", () => {
 
   beforeEach(() => {
     tmpDir = makeTmpDir();
-    kbDir = join(tmpDir, ".kb");
+    kbDir = join(tmpDir, ".fusion");
   });
 
   afterEach(async () => {
@@ -77,7 +77,7 @@ describe("getMigrationStatus", () => {
 
   beforeEach(() => {
     tmpDir = makeTmpDir();
-    kbDir = join(tmpDir, ".kb");
+    kbDir = join(tmpDir, ".fusion");
   });
 
   afterEach(async () => {
@@ -121,7 +121,7 @@ describe("migrateFromLegacy", () => {
 
   beforeEach(async () => {
     tmpDir = makeTmpDir();
-    kbDir = join(tmpDir, ".kb");
+    kbDir = join(tmpDir, ".fusion");
     await mkdir(kbDir, { recursive: true });
     db = new Database(kbDir);
     db.init();