chore: migrate to pnpm for improved monorepo management (#287)

# Migrate to pnpm for improved monorepo management

## Summary
This PR migrates the Codex repository from npm to pnpm, providing faster
dependency installation, better disk space usage, and improved monorepo
management.

## Changes
- Added `pnpm-workspace.yaml` to define workspace packages
- Added `.npmrc` with optimal pnpm configuration
- Updated root package.json with workspace scripts
- Moved resolutions and overrides to the root package.json
- Updated scripts to use pnpm instead of npm
- Added documentation for the migration
- Updated GitHub Actions workflow for pnpm

## Benefits
- **Faster installations**: pnpm is significantly faster than npm
- **Disk space savings**: pnpm's content-addressable store avoids
duplication
- **Strict dependency management**: prevents phantom dependencies
- **Simplified monorepo management**: better workspace coordination
- **Preparation for Turborepo**: as discussed, this is the first step
before adding Turborepo

## Testing
- Verified that `pnpm install` works correctly
- Verified that `pnpm run build` completes successfully
- Ensured all existing functionality is preserved

## Documentation
Added a detailed migration guide in `PNPM_MIGRATION.md` explaining:
- Why we're migrating to pnpm
- How to use pnpm with this repository
- Common commands and workspace-specific commands
- Monorepo structure and configuration

## Next Steps
As discussed, once this change is stable, we can consider adding
Turborepo as a follow-up enhancement.

codex-cli/package.json

@@ -22,8 +22,8 @@
     "build:dev": "NODE_ENV=development node build.mjs --dev && NODE_OPTIONS=--enable-source-maps node dist/cli-dev.js",
     "release:readme": "cp ../README.md ./README.md",
     "release:version": "TS=$(date +%y%m%d%H%M) && sed -E -i'' -e \"s/\\\"0\\.1\\.[0-9]{10}\\\"/\\\"0.1.${TS}\\\"/g\" package.json src/utils/session.ts",
-    "release:build-and-publish": "npm run build && npm publish",
-    "release": "npm run release:readme && npm run release:version && npm install && npm run release:build-and-publish",
+    "release:build-and-publish": "pmpm run build && pmpm publish",
+    "release": "pnpm run release:readme && pnpm run release:version && pnpm install && pnpm run release:build-and-publish",
     "prepare": "husky",
     "pre-commit": "lint-staged"
   },
@@ -79,14 +79,6 @@
     "vitest": "^3.0.9",
     "whatwg-url": "^14.2.0"
   },
-  "resolutions": {
-    "braces": "^3.0.3",
-    "micromatch": "^4.0.8",
-    "semver": "^7.7.1"
-  },
-  "overrides": {
-    "punycode": "^2.3.1"
-  },
   "repository": {
     "type": "git",
     "url": "https://github.com/openai/codex"