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>

AGENTS.md

@@ -14,9 +14,9 @@
 主入口是 `EmbeddedAnimPacker.py`，提供两种用法：
 
 - **命令行**：传入 GIF 与参数，行为与脚本最初版本一致。
-- **交互式菜单**：不带 GIF 参数（或传 `-i`/`--interactive`）时进入，基于 `questionary` 选择素材并配置参数。
+- **交互式菜单**：不带 GIF 参数（或传 `-i`/`--interactive`）时进入，基于 `questionary` 选择素材并配置参数。`_select_gifs()` 采用两步式选择，返回 GIF 列表，支持多选 / 一键全选 / 手动输入路径。
 
-两条路径都汇聚到同一个 `run_conversion(ConversionConfig)` 管线，避免逻辑重复。
+两条路径都汇聚到同一个 `run_conversion(ConversionConfig)` 单 GIF 管线。命令行直接调用它；菜单经 `run_conversions(gifs, template)` 逐个调用（continue-on-error，末尾打印聚合结果），且多选时强制用各 GIF 自身 stem 作帧前缀以避免冲突。
 
 ## 重要文件