feat: restructure as pnpm monorepo workspace

- Move limit-turns.ts into packages/pi-turn-limit/src/index.ts
- Add root package.json (private workspace manifest)
- Add pnpm-workspace.yaml
- Add shared tsconfig.json
- Add pi package manifest to extension (pi.extensions, pi-package keyword)
- Update AGENTS.md with monorepo workflow

.gitignore

@@ -0,0 +1,3 @@
+node_modules/
+.pnpm-store/
+pnpm-lock.yaml