Previously the hook redirected stderr to /dev/null, hiding useful error
messages like 'Git conflict markers detected'. Users only saw a generic
warning with no indication of the root cause.
Now captures and displays the actual error output, making it immediately
clear what went wrong (e.g., merge conflicts in JSONL).
Fixes bd-nszi
Amp-Thread-ID: https://ampcode.com/threads/T-6231e968-2c06-44ad-855d-bb5fafe452f6
Co-authored-by: Amp <amp@ampcode.com>