chore: remove temp files from git and add to .gitignore

The bump workflow generates temporary files (commit-message.txt,
pr-body.txt, bump-output.txt) that should never be tracked in git.
While the workflow cleanup step (c8d5e4f) removes these files
after use, they were already committed to the repository.

This commit:
- Removes the files from git tracking
- Adds them to .gitignore to prevent future accidental commits

This prevents PRs like #3160 which only delete these temp files.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: 2 people

.gitignore

@@ -90,3 +90,8 @@ test/e2e/common/cosign.pub
 
 # downloaded release files during build
 /cmd/*/operator/kodata/
+
+# Workflow temporary files
+commit-message.txt
+pr-body.txt
+bump-output.txt