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>

.gitignore

@@ -33,5 +33,5 @@ Thumbs.db
 # ── u-claw-app runtime binaries (download on build) ──
 u-claw-app/resources/runtime/
 
-# ── Bootable USB (moved to separate repo: u-claw-linux) ──
-bootable/
+# ── Bootable USB build artifacts (scripts are tracked, downloads are not) ──
+bootable/.download-cache/