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/openclaw.desktop

@@ -0,0 +1,7 @@
+[Desktop Entry]
+Name=U-Claw AI Assistant
+Comment=OpenClaw AI - Plug and Play
+Exec=/opt/u-claw/start-openclaw.sh
+Terminal=true
+Type=Application
+Categories=Utility;