Add sandbox symlink support

[massivemoose]

As titled. Many agents (Codex, Claude, etc.) do not automatically allow writing through symlinks. That's obviously a problem. This provides a few things:

1. Clear messaging in the CLI about setting up agents for a more seamless experience.
2. A framework to add support for agents pretty easily. I've added Claude and Codex, but adding additional ones via a new file are pretty trivial for people that want to add support for them.
3. Clear docs in the repo, with links from help messages as well.