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.

PNPM_MIGRATION.md

@@ -0,0 +1,70 @@
+# Migration to pnpm
+
+This project has been migrated from npm to pnpm to improve dependency management and developer experience.
+
+## Why pnpm?
+
+- **Faster installation**: pnpm is significantly faster than npm and yarn
+- **Disk space savings**: pnpm uses a content-addressable store to avoid duplication
+- **Phantom dependency prevention**: pnpm creates a strict node_modules structure
+- **Native workspaces support**: simplified monorepo management
+
+## How to use pnpm
+
+### Installation
+
+```bash
+# Global installation of pnpm
+npm install -g pnpm@10.8.1
+
+# Or with corepack (available with Node.js 22+)
+corepack enable
+corepack prepare pnpm@10.8.1 --activate
+```
+
+### Common commands
+
+| npm command     | pnpm equivalent  |
+| --------------- | ---------------- |
+| `npm install`   | `pnpm install`   |
+| `npm run build` | `pnpm run build` |
+| `npm test`      | `pnpm test`      |
+| `npm run lint`  | `pnpm run lint`  |
+
+### Workspace-specific commands
+
+| Action                                     | Command                                  |
+| ------------------------------------------ | ---------------------------------------- |
+| Run a command in a specific package        | `pnpm --filter @openai/codex run build`  |
+| Install a dependency in a specific package | `pnpm --filter @openai/codex add lodash` |
+| Run a command in all packages              | `pnpm -r run test`                       |
+
+## Monorepo structure
+
+```
+codex/
+├── pnpm-workspace.yaml    # Workspace configuration
+├── .npmrc                 # pnpm configuration
+├── package.json           # Root dependencies and scripts
+├── codex-cli/             # Main package
+│   └── package.json       # codex-cli specific dependencies
+└── docs/                  # Documentation (future package)
+```
+
+## Configuration files
+
+- **pnpm-workspace.yaml**: Defines the packages included in the monorepo
+- **.npmrc**: Configures pnpm behavior
+- **Root package.json**: Contains shared scripts and dependencies
+
+## CI/CD
+
+CI/CD workflows have been updated to use pnpm instead of npm. Make sure your CI environments use pnpm 10.8.1 or higher.
+
+## Known issues
+
+If you encounter issues with pnpm, try the following solutions:
+
+1. Remove the `node_modules` folder and `pnpm-lock.yaml` file, then run `pnpm install`
+2. Make sure you're using pnpm 10.8.1 or higher
+3. Verify that Node.js 22 or higher is installed