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/ventoy/ventoy.json

@@ -0,0 +1,9 @@
+{
+  "persistence": [
+    {
+      "image": "/ubuntu-24.04.2-desktop-amd64.iso",
+      "backend": "/persistence.dat",
+      "autosel": 1
+    }
+  ]
+}