feat: add interactive configuration menu

Running without a GIF argument (or with -i/--interactive) now launches a
questionary-based menu that selects a source GIF from the current directory
and configures every conversion parameter before running. Passing a GIF on
the command line behaves exactly as before.

- extract a unified run_conversion(ConversionConfig) pipeline shared by the
  CLI and the menu, removing the duplicated convert loop in main()
- make the gif positional optional and add -i/--interactive
- import questionary lazily inside the menu so plain CLI usage gains no hard
  dependency, with a clear install hint when it is missing
- add __version__ / --version (0.2.0) and document the menu in README/AGENTS

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -11,13 +11,25 @@ python3 -m venv .venv
 . .venv/bin/activate
 ```
 
-然后安装 GIF 读取依赖：
+然后安装依赖（Pillow 用于读取 GIF，questionary 用于交互式菜单）：
 
 ```bash
 python3 -m pip install -r requirements.txt
 ```
 
-然后传入 GIF 文件：
+## 交互式菜单
+
+不带任何参数运行，会进入交互式菜单：可以从当前目录选择源 GIF、逐项配置参数，最后确认并执行转换：
+
+```bash
+python3 EmbeddedAnimPacker.py
+```
+
+也可以用 `-i` / `--interactive` 显式进入菜单。菜单仅在进入时才需要 `questionary`；纯命令行用法不依赖它。
+
+## 命令行
+
+直接传入 GIF 文件即可（行为与以前一致）：
 
 ```bash
 python3 EmbeddedAnimPacker.py boot.gif