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>

.gitignore

@@ -32,3 +32,6 @@ 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/