feat: support multi-select / select-all of source GIFs in the menu

The menu's source step now uses a two-step selection (_select_gifs): pick a
mode — tick several GIFs (questionary.checkbox), all GIFs in the directory at
once, or enter a path manually — then convert them in one batch. Single-GIF
selection and the entire command line stay unchanged.

- add run_conversions(gifs, template) that reuses the unchanged single-GIF
  run_conversion() per item, continues on per-GIF errors, and prints an
  aggregate "Batch complete: X/Y" summary (non-zero exit if any failed)
- with multiple GIFs the per-GIF stem is forced as the frame prefix so frames
  never collide; the prefix prompt is only shown for a single GIF
- update README/AGENTS for the multi/all selection and batch behavior

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

README.md

@@ -19,7 +19,7 @@ python3 -m pip install -r requirements.txt
 
 ## 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：
+不带任何参数运行，会进入交互式菜单：从当前目录选择源 GIF（支持**多选或一键全选**，批量转换）、逐项配置参数，最后确认并执行转换：
 
 ```bash
 python3 EmbeddedAnimPacker.py