Replace String() calls with .toString()/.toLocaleString() per ast-grep rule

- Add formatError() helper in CLI to safely handle unknown error types
- Add uniqueTestId() helper for generating unique test identifiers
- Replace String(id) with id.toString() for database ID conversions
- Replace String(n) with n.toLocaleString() for user-facing number formatting
- Fix TypeScript errors in test files (undefined checks, unused variables)
- Update lint commands to include ast-grep scanning

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -11,8 +11,8 @@
     "build": "turbo build",
     "build:watch:packages": "turbo watch build --filter=./packages/*",
     "build:packages": "turbo build --filter=./packages/*",
-    "lint": "biome check && turbo run lint",
-    "lint:fix": "biome check --write --unsafe && turbo run lint -- --fix",
+    "lint": "biome check && ast-grep scan && turbo run lint",
+    "lint:fix": "biome check --write --unsafe && ast-grep scan --update-all && turbo run lint -- --fix",
     "typecheck": "turbo typecheck",
     "clean": "turbo clean",
     "test": "turbo test",