feat: add bootable Linux USB module (independent within repo)

Add bootable/ directory with complete toolkit to create a bootable
Ubuntu USB drive with OpenClaw AI:
- 4-step PowerShell scripts for USB preparation on Windows
- Ventoy 1.0.99 boot manager with casper-rw persistence
- Self-contained Linux setup and startup scripts
- Detailed README with architecture, usage guide, and troubleshooting

bootable/ is fully independent - does not reference any files from
portable/, u-claw-app/, or website/. Also maintained as standalone
repo at u-claw-linux for convenience.

Update main README to reference both bootable/ and u-claw-linux repo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bootable/linux-setup/start-openclaw.sh

@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+# ============================================================
+#  U-Claw OpenClaw Startup Script (Linux)
+#  Completely self-contained — no external file dependencies
+# ============================================================
+
+set -euo pipefail
+
+INSTALL_DIR="/opt/u-claw"
+NODE_BIN="$INSTALL_DIR/runtime/node-linux-x64/bin/node"
+CORE_DIR="$INSTALL_DIR/core"
+DATA_DIR="$INSTALL_DIR/data"
+
+export OPENCLAW_HOME="$DATA_DIR/.openclaw"
+export OPENCLAW_STATE_DIR="$DATA_DIR/.openclaw"
+export OPENCLAW_CONFIG_PATH="$DATA_DIR/.openclaw/openclaw.json"
+
+# --- Sanity checks ---
+if [[ ! -x "$NODE_BIN" ]]; then
+    echo "[ERROR] Node.js not found at $NODE_BIN"
+    echo "Please run setup-openclaw.sh first."
+    read -rp "Press Enter to exit..."
+    exit 1
+fi
+
+if [[ ! -d "$CORE_DIR/node_modules/openclaw" ]]; then
+    echo "[ERROR] OpenClaw not installed in $CORE_DIR"
+    echo "Please run setup-openclaw.sh first."
+    read -rp "Press Enter to exit..."
+    exit 1
+fi
+
+# --- Find available port in 18789-18799 ---
+PORT=""
+for p in $(seq 18789 18799); do
+    if ! ss -tlnp 2>/dev/null | grep -q ":${p} "; then
+        PORT="$p"
+        break
+    fi
+done
+
+if [[ -z "$PORT" ]]; then
+    echo "[ERROR] All ports 18789-18799 are in use."
+    read -rp "Press Enter to exit..."
+    exit 1
+fi
+
+echo "============================================"
+echo "  U-Claw AI Assistant"
+echo "  Starting on port $PORT ..."
+echo "============================================"
+
+# --- Check if model is configured ---
+CONFIG_FILE="$OPENCLAW_CONFIG_PATH"
+FIRST_RUN=false
+if [[ ! -f "$CONFIG_FILE" ]] || ! grep -q '"apiKey"' "$CONFIG_FILE" 2>/dev/null; then
+    FIRST_RUN=true
+    echo ""
+    echo "[INFO] No AI model configured yet."
+    echo "       After startup, please configure your model in the browser."
+    echo ""
+fi
+
+# --- Start OpenClaw gateway ---
+cd "$CORE_DIR"
+OPENCLAW_ENTRY=$(find node_modules/openclaw -name "openclaw.mjs" -maxdepth 2 2>/dev/null | head -1)
+
+if [[ -z "$OPENCLAW_ENTRY" ]]; then
+    echo "[ERROR] Cannot find openclaw.mjs entry point."
+    read -rp "Press Enter to exit..."
+    exit 1
+fi
+
+echo "Opening browser at http://localhost:$PORT ..."
+
+# Open browser after a short delay
+(sleep 3 && xdg-open "http://localhost:$PORT" 2>/dev/null) &
+
+"$NODE_BIN" "$OPENCLAW_ENTRY" gateway run \
+    --allow-unconfigured \
+    --force \
+    --port "$PORT"