diigo-cache: own test toolchain; pnpm lint, pnpm test

vitest, @cloudflare/vitest-plugin, and typescript are devDependencies
here instead of being borrowed from diigo-bookmarks through hand-made
node_modules symlinks and a deep-path import.

- lint: wrangler types, then tsc over src/ and tests/ (Cloudflare's
  documented typecheck shape: regenerate types, then tsc)
- test: vitest run
- tests/tsconfig.json per the vitest-plugin docs; the dangling
  typecheck script (tsconfig.check.json never existed) is gone

Generated-by: Claude Opus 5.5

diigo-cache/tests/tsconfig.json

@@ -0,0 +1,8 @@
+{
+	"extends": "../tsconfig.json",
+	"compilerOptions": {
+		"types": ["@cloudflare/vitest-plugin/types"],
+		"allowJs": true
+	},
+	"include": ["./**/*", "../worker-configuration.d.ts"]
+}