fix(impl-merge): clean up stale impl:LIB:failed label on successful merge

When impl-generate.yml fails (e.g. environment setup error) it tags the
parent issue with `impl:LIB:failed`. A subsequent successful retry merges
through impl-merge.yml, which previously only removed `impl:LIB:pending`
and added `impl:LIB:done` — leaving the stale `:failed` label in place.

Observed on #3802 (skewt-logp-atmospheric/makie): first generate failed
on Project.toml setup (added :failed), retry succeeded after PR #7614
(added :done), and the issue closed with both labels for the same lib.

The close-check counts in this same workflow use if/elif (done wins over
failed) so totals weren't actually broken — but the labels were a lie.
Other readers (DB sync, UI, future workflows) could trust them less
charitably, so remove the stale label at the canonical merge point.

Author: MarkusNeusinger

.github/workflows/impl-merge.yml

@@ -292,9 +292,14 @@ jobs:
           # Create labels if they don't exist
           gh label create "impl:${LIBRARY}:done" --color "0e8a16" --description "${LIBRARY} implementation merged" 2>/dev/null || true
 
-          # Remove trigger and pending, add done
+          # Remove trigger, pending, and any stale failed label from earlier attempts,
+          # then add done. The :failed label is added by impl-generate.yml when an
+          # attempt errors out (e.g. environment setup failure); a later successful
+          # retry merges via this workflow but historically didn't clean up :failed,
+          # leaving the issue with both :done and :failed for the same library.
           gh issue edit "$ISSUE" --remove-label "generate:${LIBRARY}" 2>/dev/null || true
           gh issue edit "$ISSUE" --remove-label "impl:${LIBRARY}:pending" 2>/dev/null || true
+          gh issue edit "$ISSUE" --remove-label "impl:${LIBRARY}:failed" 2>/dev/null || true
           gh issue edit "$ISSUE" --add-label "impl:${LIBRARY}:done"
 
       - name: Post success to issue