fix: symlink gh config in dev-env.js to preserve auth on Unix

[maxtara]

Summary

Running npm run dev on Linux/macOS caused Copilot SDK authentication to fail because dev-env.js overrides XDG_CONFIG_HOME, which breaks gh CLI ability to find its auth token.

Root Cause

scripts/dev-env.js sets XDG_CONFIG_HOME=~/.cooper-dev, but gh CLI follows the XDG spec and looks for config at $XDG_CONFIG_HOME/gh/hosts.yml instead of the default ~/.config/gh/hosts.yml.

Fix

On Unix systems, automatically symlink ~/.config/gh to ~/.cooper-dev/gh before overriding XDG_CONFIG_HOME. This preserves dev isolation while allowing gh to find its auth token.

Windows is unaffected since it ignores XDG_CONFIG_HOME entirely.

Testing

• All 395 unit tests pass
• Requires manual verification on Linux/macOS:
  1. Remove any existing symlink: rm ~/.cooper-dev/gh
  2. Run npm run dev
  3. Confirm auth works and chat responds