Add GitHub CLI to setup script

Install gh (GitHub CLI) via Homebrew and prompt for authentication if not already logged in. This enables GitHub automation and CLI workflows during initial setup.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

setup.sh

@@ -23,6 +23,7 @@ PACKAGES=(
     fish
     dust
     eza
+    gh
     htop
     go
     jq
@@ -68,6 +69,18 @@ else
     echo "Skipping macOS-specific casks (not on macOS)"
 fi
 
+# Set up GitHub CLI authentication
+if command -v gh &> /dev/null; then
+    if ! gh auth status &>/dev/null; then
+        echo ""
+        echo "GitHub CLI is installed but not authenticated."
+        echo "Please log in to GitHub CLI:"
+        gh auth login
+    else
+        echo "✓ GitHub CLI already authenticated"
+    fi
+fi
+
 echo "✓ Setup complete!"
 echo ""
 echo "Next steps:"