build: share root config with pi-private

Everything outside apps/ is the same in pi and pi-private; apps/ exists only
in pi-private, and these files now tolerate its absence:
- workspaces include apps/*
- biome.json leaves apps/ to its own config
- the pre-commit hook runs the apps/mobile check only when it exists
- the Android APK workflow runs only in earendil-works/pi-private
- scripts/biome-check.mjs runs native Biome, or Biome WASM on Android
  (@biomejs/wasm-nodejs 2.3.5), for npm run check
From pi-private bac58de97, c2a8629da, 106714ebc.

.husky/pre-commit

@@ -16,6 +16,24 @@ if [ $? -ne 0 ]; then
   exit 1
 fi
 
+# Pi Mobile has its own check (own biome.json, TypeScript 5.9; apps/ is outside the root Biome and tsc configs).
+# apps/ exists only in pi-private.
+if [ -d apps/mobile ]; then
+  for file in $STAGED_FILES; do
+    case "$file" in
+      apps/mobile/*|packages/*|package.json|package-lock.json|tsconfig.json|tsconfig.base.json)
+        echo "Checking apps/mobile..."
+        npm --prefix apps/mobile run check
+        if [ $? -ne 0 ]; then
+          echo "❌ apps/mobile checks failed."
+          exit 1
+        fi
+        break
+        ;;
+    esac
+  done
+fi
+
 RUN_BROWSER_SMOKE=0
 for file in $STAGED_FILES; do
   case "$file" in