docs: add Linux bootable USB as separate repo (u-claw-linux)

- Add links to u-claw-linux repo in README (both CN and EN sections)
- Add Linux x64 to platform support table
- Update CLAUDE.md platform status
- Move bootable/ to .gitignore (now independent repo)

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

CLAUDE.md

@@ -90,4 +90,4 @@ Release artifacts go to GitHub Releases, not the repo.
 - Mac Apple Silicon (ARM64): ✅ Working
 - Mac Intel (x64): ✅ Working（portable 需先运行 setup.sh 下载 node-mac-x64）
 - Windows x64: 🚧 In development
-- Linux: 不提供脚本，后续只提供教程
+- Linux (Bootable USB): ✅ Separate repo → [u-claw-linux](https://github.com/dongsheng123132/u-claw-linux)