Handle errors during git rebase with grace

```
⬢ [Docker] ❯ git rebase --continue
...
PartCAD: Prepare Commit Message..........................................Failed
- hook id: prepare-commit-msg
- exit code: 1

/workspaces/partcad/.pre-commit/repo818bfy_u/prepare-commit-msg.sh: line 205: OPENROUTER_API_KEY: unbound variable

check yaml...........................................(no files to check)Skipped
```