feat: add Linux support, pnpm build fixes, and website updates

- Add Linux x64 platform support (运行.sh, build script, docs)
- Fix pnpm-first build logic across all launcher scripts
- Update website: 3→4 platforms, Linux in 3-step guide
- Update docs: Qwen free tier, StepFun model, 省钱提示
- Fix .gitignore for build artifacts

.gitignore

@@ -1,9 +1,5 @@
-# U-Claw build outputs (too large for git)
-U-Claw/runtime/
-U-Claw/openclaw/node_modules/
-U-Claw/openclaw/dist/
-U-Claw/openclaw/.git/
-U-Claw/backups/
+# U-Claw build output bundle (generated release artifact)
+U-Claw/
 
 # OpenClaw source (users download separately)
 openclaw-2026.3.7/