ci: read platform from MokoGitea metadata API in pr-check (retire manifest.xml)

Matches Template-Generic's dev block. Platform detection now GETs
/api/v1/repos/{owner}/{repo}/metadata instead of reading .mokogitea/manifest.xml;
falls back to 'generic' if unavailable.

.mokogitea/workflows/pr-check.yml

@@ -149,9 +149,9 @@ jobs:
       - name: Detect platform
         id: platform
         run: |
-          # Read platform from XML manifest (<platform> tag) or plain text fallback
-          PLATFORM=$(sed -n 's/.*<platform>\([^<]*\)<\/platform>.*/\1/p' .mokogitea/manifest.xml 2>/dev/null | head -1)
-          [ -z "$PLATFORM" ] && PLATFORM=$(cat .mokogitea/manifest.xml 2>/dev/null | tr -d '[:space:]')
+          # Platform comes from the MokoGitea metadata API (public GET).
+          API="${GITHUB_SERVER_URL:-https://git.mokoconsulting.tech}/api/v1/repos/${GITHUB_REPOSITORY}/metadata"
+          PLATFORM="$(curl -sf "$API" 2>/dev/null | python3 -c "import sys, json; print(json.load(sys.stdin).get('platform') or '')" 2>/dev/null || true)"
           [ -z "$PLATFORM" ] && PLATFORM="generic"
           echo "platform=$PLATFORM" >> "$GITHUB_OUTPUT"