ops: address argus review findings on PR #4433

Add attribution-trailer lint row to the ci.md checks table (modeled
on the vale-errors row: not yet a required check) so the doc doesn't
assert stale completeness once attribution-trailers merges. Delete
the dead CI_COMMIT_REF_NAME elif in check-attribution-trailers.sh —
byte-identical to the else arm, never exercised, and its comment
overclaimed GitLab/Forgejo support nothing here can test.

Refs #4432

scripts/check-attribution-trailers.sh

@@ -12,8 +12,6 @@ set -eu
 
 if [ -n "${GITHUB_BASE_REF:-}" ]; then
   base="origin/${GITHUB_BASE_REF}"
-elif [ -n "${CI_COMMIT_REF_NAME:-}" ]; then
-  base="origin/main"
 else
   base="origin/main"
 fi