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
