feat: add runtime-image-select skill

Base runtime image selection and Dockerfile generation/audit per the
runtime-base image matrix, packaged as a skills.sh-compliant skill.

README.md

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+# Images Skills
+
+[Agent Skills](https://skills.sh) 集合,可安装到 Claude Code、Cursor、Codex 等支持 skill 的 Agent 中。
+
+## 安装
+
+安装全部 skills:
+
+```bash
+npx skills add gitea@livebook:images/skills.git
+```
+
+只安装指定 skill:
+
+```bash
+npx skills add gitea@livebook:images/skills.git --skill runtime-image-select
+```
+
+## Skills 列表
+
+| Skill | 说明 |
+| --- | --- |
+| [runtime-image-select](skills/runtime-image-select/SKILL.md) | 探测项目技术栈,按 runtime-base 镜像矩阵选型并生成/修正 Dockerfile |
+
+## 目录结构
+
+每个 skill 位于 `skills/<name>/` 目录下,包含一个带 `name` 与 `description` frontmatter 的 `SKILL.md`:
+
+```
+skills/
+└── runtime-image-select/
+    ├── SKILL.md          # skill 主体(必需)
+    ├── references/       # 补充参考文档
+    └── scripts/          # 辅助脚本
+```
+
+## License
+
+[MIT](LICENSE)