chore(theme): astro 锁定 ~5.18.0（终审 Minor）+ docs: Plan 4 收官

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

themes/pilot-minimal/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@sf/theme-pilot-minimal",
   "version": "0.1.0",
-  "description": "SiteForge pilot 极简主题：纯静态 Astro 5，渲染数据唯一入口 load-render-data（五禁纪律的结构保证）",
+  "description": "SiteForge pilot \u6781\u7b80\u4e3b\u9898\uff1a\u7eaf\u9759\u6001 Astro 5\uff0c\u6e32\u67d3\u6570\u636e\u552f\u4e00\u5165\u53e3 load-render-data\uff08\u4e94\u7981\u7eaa\u5f8b\u7684\u7ed3\u6784\u4fdd\u8bc1\uff09",
   "type": "module",
   "scripts": {
     "build": "astro build",
@@ -9,7 +9,7 @@
     "typecheck": "tsc -p tsconfig.json --noEmit"
   },
   "dependencies": {
-    "astro": "^5.0.0"
+    "astro": "~5.18.0"
   },
   "devDependencies": {
     "@sf/contracts": "workspace:*",