chore: gitignore tmp/ + remove accidentally-committed audit screenshots

The 100 PNGs from the in-progress visual audit pass landed in cb91f78
because tmp/ wasn't gitignored. Removing from HEAD + adding the rule
so future runs stay local. (Original blobs remain reachable from the
prior commit if needed; not worth a destructive filter-branch.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -58,3 +58,6 @@ docker-compose.override.yml
 
 # Local berth-PDF + brochure samples used as upload fixtures during dev.
 /berth_pdf_example/
+
+# Scratch / audit artefacts
+tmp/