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/cli/src/bin.ts

@@ -28,7 +28,7 @@ if (isBunBinary) {
     writeFileSync(
       join(tmp, "package.json"),
       JSON.stringify(
-        { name: "kb", version: "0.1.0", type: "module", piConfig: { name: "kb", configDir: ".kb" } },
+        { name: "kb", version: "0.1.0", type: "module", piConfig: { name: "kb", configDir: ".fusion" } },
         null,
         2,
       ) + "\n",