Summary

Fix the DCO Assistant workflow so the comment it asks first-time contributors to post also triggers signature processing. Today, contributors can work around the problem by posting a second recheck comment, but the documented one-comment signing flow does not work.

PR #2395 exposed the issue: its commit contains a valid Signed-off-by trailer and the contributor posted the requested DCO signature comment, but the comment-triggered workflow skipped its only action step and left the original check failing.

Related Issue

Observed on #2395.

Changes

• Define the requested DCO signature text once as DCO_SIGN_COMMENT.
• Use that value both in the workflow trigger and in custom-pr-sign-comment.
• Remove the stale CLA-oriented trigger text, which does not match the DCO comment shown to contributors.

Why This Is Needed

The current workflow asks contributors to post:

I have read the DCO document and I hereby sign the DCO.

However, its step condition expects:

I have read the Contributor Agreement including DCO and I hereby sign the Contributor Agreement and DCO

Because those values differ, posting the requested signature starts an issue_comment workflow run but skips the DCO Assistant step. A subsequent recheck comment happens to work because it triggers the action, which scans the earlier comments, finds the DCO signature, records the user in dco-signatures.json, and reruns the failed check.

Recent first-time contributors have passed through this two-comment workaround, including #2367, #2315, #2319, #2324, #2285, and #2250. Contributors already present in dco-signatures.json pass without encountering the bug.

The upstream action documents a one-comment flow: the configured signature comment must also appear in the workflow condition. This change restores that intended behavior. It does not alter the DCO policy or signature text, and recheck remains available for manual retriggering.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not applicable)
• Compared the workflow condition with custom-pr-sign-comment
• Verified docs(prover): correct prove() exit code doc comment #2395 skipped the DCO Assistant step after the requested signature comment
• Verified recent first-time signers passed after posting recheck

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable)