chore: migrate from npm to pnpm

- Replace package-lock.json with pnpm-lock.yaml (pnpm 11.7.0)
- Add packageManager field to package.json
- Add pnpm-workspace.yaml with approved build scripts (esbuild, unrs-resolver)
- Verified: 25/25 tests pass, lint clean, static build OK

pnpm-workspace.yaml

@@ -0,0 +1,6 @@
+allowBuilds:
+  esbuild: true
+  unrs-resolver: true
+onlyBuiltDependencies:
+  - esbuild
+  - unrs-resolver