feat: add git post-receive hook for auto build and start

Adds a `git_hooks/post-receive` script that runs `pnpm run build:start`
(equivalent to `next build && next start`) automatically after a push is
received, enabling deployment on `receive.denyCurrentBranch=updateInstead`
configured repos (e.g., production/test servers).

Updates the README with a new "Git Hooks" section explaining:

- How to enable the hooks locally via `git config core.hooksPath git_hooks`
- That `core.hooksPath` is a local config and must be set per clone
- That `post-receive` fires on the receiving (push target) machine, not
  the local dev machine

git_hooks/post-receive

@@ -0,0 +1,11 @@
+#!/bin/sh
+# Post-receive hook: push 后自动 build + start
+#
+# 触发场景：
+#   - 非 bare repo + receive.denyCurrentBranch=updateInstead：本地推送自动部署
+#
+# 用途：执行 package.json 中的 `build:start` 脚本
+#   - 内部 = `npm run build && npm run start`
+#   - 实际调用 `pnpm run build:start`（项目使用 pnpm）
+
+pnpm run build:start