docs(plane): 自托管 Plane 落地事实入仓库 + plane-health runbook + hardening 草稿

记录源 Linear→Plane (2026-09-03 起, Plane MCP) + plane.chans.xyz 服务行/upstream 段;
inventory + hosts/synapse.chans.xyz.md 补 Plane 部署事实 (Helm plane-ce-1.8.0 / app v1.4.1,
ns plane, IngressRoute/自有证书 issuer/PVC 5+5Gi local-path/无备份层);
新增 runbooks/plane-health.md (只读健康检查) 与 docs/plane-hardening/ 草稿
(values.hardened.yaml、secrets.yaml.example 占位、backup/ CronJob), 均为未应用设计稿;
.gitignore 增加 .tmp-* agent 临时文件。

.gitignore

@@ -32,3 +32,4 @@ skills-lock.json
 *~
 # Agent working scratch (not repo content).
 .agent-work/
+.tmp-*