Add `/add-dir` slash command to add directories mid-session

Feature Request

Problem

Codex CLI supports --add-dir as a startup flag, but there's no way to add additional working directories during an active session. If you realize mid-conversation that you need the agent to access another directory, you have to quit, restart with the extra --add-dir flag, and lose your session context.

Claude Code solves this with a /add-dir slash command that lets you add directories on the fly without restarting.

Proposed Solution

Add a /add-dir slash command that can be used mid-session to expand the agent's accessible directories.

Example usage (during a session):

> /add-dir ../frontend
Added directory: /Users/me/projects/frontend

> /add-dir ../shared-lib /tmp/fixtures
Added directories:
  - /Users/me/projects/shared-lib
  - /tmp/fixtures

Use Cases

1. Iterative exploration — You start working on one package, then realize you need to check or modify a related directory
2. Debugging across boundaries — Mid-investigation, you discover the bug lives in a different repo/directory
3. Avoiding session restarts — Preserving conversation context and progress instead of quitting and re-launching with --add-dir
4. Dynamic workflows — Projects where the set of relevant directories isn't known upfront

Behavior Details

• Should accept one or more relative or absolute paths
• Validate that each path exists and is a directory
• The agent should immediately be able to read, search, and write in newly added directories
• Could show currently active directories via /list-dirs or similar
• Optionally support /remove-dir to revoke access

Willingness to Contribute

I'd be happy to work on this and submit a PR if the maintainers are open to it. Would appreciate any pointers on where the slash command system is implemented.