fix(config): preserve explicit git_master overrides during merge

Prevent project config defaults from re-enabling git_master fields that users explicitly disabled. Rebuild git_master from explicit user and project keys so include_co_authored_by and commit_footer merge predictably.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>