docs(hooks): fix co-authorship hook JSON escaping bug

[factory-ben]

Problem

The co-authorship hook example in the git-workflows documentation had critical bugs that would prevent it from working:

1. Literal newlines in JSON output - The modified_msg variable contained actual newlines, which broke the JSON structure
2. Broken sed command - The sed replacement tried to embed multiline strings with special characters directly, causing command failure
3. Shell expansion issues - Variable expansion with newlines corrupted the final JSON output

Solution

Fixed by using jq -n with --arg to properly handle string escaping and JSON construction:

• Use \n\n for newlines in bash variable instead of literal newlines
• Extract and rebuild the git command without trying to embed complex multiline strings in sed
• Let jq handle all JSON escaping automatically

This ensures the hook will work correctly when adding co-author trailers to git commits.