docs(deploy): record production install on the Plane host behind Tailscale Serve

Container plane-mcp (network host, restart unless-stopped) on the Plane
server, endpoint https://ubuntu.<tailnet>.ts.net/http/api-key/mcp, verified
with a full MCP session; desktop serve switched off. Adds the upgrade
procedure and marks the Stage 7 rollout item done.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>