fix: pin zod to exact 4.2.1 to fix tsc build failure (#379)

The caret range ^4.2.1 allows npm to resolve zod 4.3.x, which has
breaking type changes against @modelcontextprotocol/sdk. Source builds
fail with TypeScript errors. Pinning to exact 4.2.1 resolves this.

See: https://github.com/tobi/qmd/issues/379

package.json

@@ -52,7 +52,7 @@
     "picomatch": "^4.0.0",
     "sqlite-vec": "^0.1.7-alpha.2",
     "yaml": "^2.8.2",
-    "zod": "^4.2.1"
+    "zod": "4.2.1"
   },
   "optionalDependencies": {
     "sqlite-vec-darwin-arm64": "^0.1.7-alpha.2",