Add security warnings about GITLAB_TOKEN written to temp file in write_temp_askpass

• Add WARNING in write_temp_askpass docstring about the insecurity of writing the GITLAB_TOKEN in plain text to a temporary file.
• Add TODO(security) comment with description of the planned fix (e.g. piping credentials via stdin/fd instead of writing to disk).
• Add security alert in README.md under "Git Authentication" section documenting this known limitation.