refactor: remove legacy kb compatibility

Drops the .kb/kb.db migration path, legacy backup filename handling, and
backward-compat test suites. Renames internal kbDir identifiers to
fusionDir and hasKbProject/isValidKbProject to their fusion equivalents.

- Remove needsCentralMigration, autoMigrateToCentral, and the
  "needs-migration" FirstRunState; checkAndMigrate and KB_SKIP_MIGRATION
  env var are gone
- Remove LEGACY_BACKUP_DIR and canonicalizeBackupDir; listBackups no
  longer matches kb-* filenames
- Delete backward-compat.test.ts and store-backward-compat.test.ts;
  update remaining tests to new 3-state first-run model

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/cli/src/commands/backup.test.ts

@@ -21,7 +21,7 @@ vi.mock("@fusion/core", () => ({
   TaskStore: vi.fn().mockImplementation(() => ({
     init: vi.fn().mockResolvedValue(undefined),
     getSettings: mockGetSettings,
-    kbDir: "/cwd/.fusion",
+    fusionDir: "/cwd/.fusion",
   })),
   createBackupManager: vi.fn(() => ({
     listBackups: mockListBackups,
@@ -60,7 +60,7 @@ describe("backup commands", () => {
       projectName: "demo-project",
       projectPath: "/projects/demo",
       isRegistered: true,
-      store: { getSettings: mockGetSettings, kbDir: "/projects/demo/.fusion" },
+      store: { getSettings: mockGetSettings, fusionDir: "/projects/demo/.fusion" },
     });
   });