refactor(cli): _serve-nitro 改用 #nitro subpath import

src/cli/_serve-nitro.mjs 原本用 ../../.output/server/index.mjs 跨边界导入 nitro 构建产物，与 #package / #drizzle/* 同属 "src/ 跳出根目录" 场景。统一改为 #nitro。

新增 package.json#imports:
  "#nitro": "./.output/server/index.mjs"

端到端验证（compose + Postgres 18）：
- 编译二进制内嵌 nitro serve() 入口 ✓
- ./server migrate：embedded SQL 应用成功 ✓
- ./server 运行：/health、/api/spec.json (title/version)、RPC create+list 全 OK ✓
- Stack trace 印证 #nitro 由 Bun 正确解析到 .output/server/index.mjs ✓
- biome/typecheck/test/build/compile 全绿

package.json

@@ -4,8 +4,9 @@
   "private": true,
   "type": "module",
   "imports": {
-    "#drizzle/*.sql": "./drizzle/*.sql",
-    "#package": "./package.json"
+    "#package": "./package.json",
+    "#nitro": "./.output/server/index.mjs",
+    "#drizzle/*.sql": "./drizzle/*.sql"
   },
   "scripts": {
     "build": "bunx --bun vite build",
@@ -65,4 +66,4 @@
     "typescript": "^6.0.3",
     "vite": "^8.0.10"
   }
-}
+}