feat: evaluate attachment and image gallery tools

[pandaria75]

背景

后续内容生产工作流可能需要管理文章附件、配图和素材。BookStack 支持 attachments 和 image gallery 等相关 API。

目标

评估是否需要新增：

• list attachments
• get attachment
• upload attachment
• update attachment
• list image gallery
• upload image
• get image metadata

非目标

本 issue 只做评估或后续实现规划，不要求第一阶段开发。

验收标准

• 梳理 BookStack 附件和图片 API 能力。
• 给出是否适合作为 Dify tool 暴露的建议。
• 明确安全限制和文件大小限制。

[pandaria75]

Completed evaluation for #45 in task planning artifacts.

Summary:

• BookStack attachments/image-gallery APIs look technically viable for future integration, but they are file-heavy and not a clean fit for the current text-first plugin surface.
• Best future fit would be narrowly scoped read-only tools, for example metadata-first or explicit-ID retrieval, not broad download/upload workflows.
• Main risks are large payload handling, accidental disclosure of private media, retention ambiguity, and expanded prompt surface through raw URLs or broad media listings.

Recommendation:

• Priority P2.
• Revisit after the core export/read tool surface is stable.
• If pursued later, start with tightly scoped read-only behavior.

No production implementation for #45 is included in this task.