Fix TypeScript config for .ts import extensions

- allowImportingTsExtensions: true + noEmit: true in tsconfig
- build script runs tsc --noEmit (type-check only)
- Dockerfile simplified to single stage using tsx at runtime
  (no tsc compilation needed; tsx handles .ts imports natively)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "main": "dist/index.js",
   "scripts": {
-    "build": "tsc",
+    "build": "tsc --noEmit",
     "dev": "tsx src/index.ts",
     "test": "vitest run",
     "test:watch": "vitest",