feat: Add Git credentials to .gitignore for security

- Prevent .git-credentials and other sensitive files from being tracked
- Add .gitconfig, *.pem, *.key to gitignore
- Ensures credential security in cross-platform development

.gitignore

@@ -221,3 +221,9 @@ __marimo__/
 desktop.ini
 Thumbs.db
 *.lnk
+
+# Git credentials (sensitive information)
+.git-credentials
+.gitconfig
+*.pem
+*.key