## Summary
- refresh git auth headers before each per-crash iteration in
`background_agent_mvp`
- add guarded `git fetch`/`git checkout` handling so one candidate auth
failure doesn’t fail the entire run
## Context
- fixes the auth failure seen in workflow run 22106378719 (`fatal: could
not read Username for 'https://github.com'`)
- related Linear issue: BIZOPS-853
## Testing
- ruby -e "require 'yaml';
YAML.load_file('.github/workflows/background_agent_mvp.yml')"
Release Notes:
- N/A
morgankrey
e0b6898ae0