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/1-prepare-usb.ps1

@@ -0,0 +1,101 @@
+# ============================================================
+#  U-Claw Bootable USB - Step 1: Prepare USB with Ventoy
+#  Downloads Ventoy and launches its installer
+# ============================================================
+
+#Requires -RunAsAdministrator
+
+$ErrorActionPreference = "Stop"
+
+Write-Host ""
+Write-Host "============================================" -ForegroundColor Cyan
+Write-Host "  U-Claw Bootable USB - Step 1: Ventoy" -ForegroundColor Cyan
+Write-Host "============================================" -ForegroundColor Cyan
+Write-Host ""
+
+# ── Config ──
+$VentoyVersion = "1.0.99"
+$VentoyUrl = "https://github.com/ventoy/Ventoy/releases/download/v${VentoyVersion}/ventoy-${VentoyVersion}-windows.zip"
+$CacheDir = Join-Path $PSScriptRoot ".download-cache"
+$VentoyZip = Join-Path $CacheDir "ventoy-${VentoyVersion}-windows.zip"
+$VentoyDir = Join-Path $CacheDir "ventoy-${VentoyVersion}"
+
+# ── Create cache directory ──
+if (-not (Test-Path $CacheDir)) {
+    New-Item -ItemType Directory -Path $CacheDir -Force | Out-Null
+}
+
+# ── List USB devices ──
+Write-Host "[INFO] Detected USB devices:" -ForegroundColor Yellow
+Write-Host ""
+$usbDisks = Get-Disk | Where-Object { $_.BusType -eq "USB" }
+if ($usbDisks.Count -eq 0) {
+    Write-Host "[ERROR] No USB devices found. Please insert a USB drive and try again." -ForegroundColor Red
+    Read-Host "Press Enter to exit"
+    exit 1
+}
+
+$usbDisks | Format-Table Number, FriendlyName, @{
+    Label = "Size (GB)"
+    Expression = { [math]::Round($_.Size / 1GB, 1) }
+}, PartitionStyle -AutoSize
+
+Write-Host ""
+Write-Host "[WARNING] Ventoy will FORMAT the selected USB drive!" -ForegroundColor Red
+Write-Host "          All data on the drive will be ERASED!" -ForegroundColor Red
+Write-Host ""
+
+$confirm = Read-Host "Type YES to continue, or anything else to cancel"
+if ($confirm -ne "YES") {
+    Write-Host "Cancelled." -ForegroundColor Yellow
+    exit 0
+}
+
+# ── Download Ventoy ──
+if (Test-Path $VentoyZip) {
+    Write-Host "[INFO] Ventoy archive already cached, skipping download." -ForegroundColor Green
+} else {
+    Write-Host "[INFO] Downloading Ventoy v${VentoyVersion}..." -ForegroundColor Yellow
+    try {
+        [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
+        $ProgressPreference = 'SilentlyContinue'
+        Invoke-WebRequest -Uri $VentoyUrl -OutFile $VentoyZip -UseBasicParsing
+        Write-Host "[OK]   Download complete." -ForegroundColor Green
+    } catch {
+        Write-Host "[ERROR] Download failed: $_" -ForegroundColor Red
+        Write-Host "        You can manually download from:" -ForegroundColor Yellow
+        Write-Host "        https://github.com/ventoy/Ventoy/releases" -ForegroundColor Yellow
+        Read-Host "Press Enter to exit"
+        exit 1
+    }
+}
+
+# ── Extract ──
+if (-not (Test-Path $VentoyDir)) {
+    Write-Host "[INFO] Extracting Ventoy..." -ForegroundColor Yellow
+    Expand-Archive -Path $VentoyZip -DestinationPath $CacheDir -Force
+    Write-Host "[OK]   Extracted." -ForegroundColor Green
+}
+
+# ── Launch Ventoy2Disk ──
+$Ventoy2Disk = Join-Path $VentoyDir "Ventoy2Disk.exe"
+if (-not (Test-Path $Ventoy2Disk)) {
+    Write-Host "[ERROR] Ventoy2Disk.exe not found at $Ventoy2Disk" -ForegroundColor Red
+    Read-Host "Press Enter to exit"
+    exit 1
+}
+
+Write-Host ""
+Write-Host "[INFO] Launching Ventoy2Disk GUI..." -ForegroundColor Yellow
+Write-Host "       1. Select your USB device in the Ventoy GUI" -ForegroundColor White
+Write-Host "       2. Click 'Install' to write Ventoy to the USB" -ForegroundColor White
+Write-Host "       3. Wait for completion, then close the GUI" -ForegroundColor White
+Write-Host ""
+
+Start-Process -FilePath $Ventoy2Disk -Wait
+
+Write-Host ""
+Write-Host "[OK]   Step 1 complete! Your USB drive now has Ventoy." -ForegroundColor Green
+Write-Host "       Next: Run .\2-download-iso.ps1" -ForegroundColor Cyan
+Write-Host ""
+Read-Host "Press Enter to continue"