refactor(auto-deploy): YAML config + post_deploy list + deploy failure mail

- New config/deploy-targets.yaml: centralized deploy target config
- Rewrite auto-deploy in _handle_pr_closed to use YAML config
- Add _send_deploy_failure_mail helper (reuses deploy_failure template)
- Support post_deploy command list (not just pm2 restart)
- Docs-only changes skip post_deploy
- Add pyyaml to pyproject.toml dependencies
- Update design doc §23 with new architecture

config/deploy-targets.yaml

@@ -0,0 +1,16 @@
+# 部署目标配置 — PR 合并后自动部署（集中管理）
+# 平台级能力，所有仓库的部署配置统一在这里维护
+
+targets:
+  sanguo/sanguo_moziplus_v2:
+    dev_dir: ~/.openclaw/sanguo_projects/sanguo_moziplus_v2
+    install_dir: ~/.sanguo_projects/sanguo_moziplus_v2
+    pm2_name: sanguo-moziplus-v2
+    rsync_exclude:
+      - .git/
+      - node_modules/
+      - __pycache__/
+      - data/
+    health_check: http://localhost:8083/health
+    post_deploy:
+      - pm2 restart sanguo-moziplus-v2