chore: switch to pnpm as the sole package manager

- Remove package-lock.json (npm) in favor of pnpm-lock.yaml
- Ignore package-lock.json and yarn.lock to prevent lockfile conflicts
  that broke module resolution in dev

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: louzoshi

.gitignore

@@ -6,6 +6,10 @@
 .pnp.js
 .yarn/install-state.gz
 
+# package manager: this project uses pnpm — keep only pnpm-lock.yaml
+package-lock.json
+yarn.lock
+
 # testing
 /coverage