cleanup: remove Linux scripts, unify core dir, fix Intel Mac bug

- Delete usb-scripts/, Linux-*.sh, setup-linux-usb.sh, migrate.js
- Rename core-mac/core-win → core (with migration shims for existing USBs)
- Fix Mac-Menu.command ARM64 hardcode — now detects Intel/Apple Silicon
- setup.sh: remove Linux branch, Windows download only with --all-platforms
- Update README.md and CLAUDE.md to reflect Mac + Windows only

portable/Mac-Diagnose.command

@@ -49,15 +49,15 @@ else
     echo -e "  ${RED}✗${NC} Node.js: NOT FOUND"
 fi
 
-# 2. Check core-mac directory
-echo "[2/6] Checking core-mac directory..."
-CORE_DIR="$UCLAW_DIR/app/core-mac"
+# 2. Check core directory
+echo "[2/6] Checking core directory..."
+CORE_DIR="$UCLAW_DIR/app/core"
 if [ -d "$CORE_DIR" ]; then
-    echo "  [OK] core-mac directory exists" >> "$LOG_FILE"
-    echo -e "  ${GREEN}✓${NC} core-mac: Found"
+    echo "  [OK] core directory exists" >> "$LOG_FILE"
+    echo -e "  ${GREEN}✓${NC} core: Found"
 else
-    echo "  [ERROR] core-mac directory not found" >> "$LOG_FILE"
-    echo -e "  ${RED}✗${NC} core-mac: NOT FOUND"
+    echo "  [ERROR] core directory not found" >> "$LOG_FILE"
+    echo -e "  ${RED}✗${NC} core: NOT FOUND"
 fi
 
 # 3. Check node_modules