Add a CI check for upstream locale keys that still show OSM wording

[Rub21]

This PR adds a CI job and a script that fail when OSM wording still shows up on the OHM site.

1. Compare config/locales/en.yml (upstream) with config/locales/overrides/en.yml (OHM).
2. Look for OSM branding in upstream text: OpenStreetMap, OSM, osm.org, State of the Map, switch2osm.
3. Skip keys the OHM site never renders, plus the ones in KEEP_UPSTREAM_WORDING where the OSM mention is correct.
4. Anything left renders on OHM, says OSM, and has no override → CI fails and prints the keys.

So when upstream adds a new label with OSM branding, we catch it before it ships.