[Requests and ideas] Allow sharing repo-associated sessions as HTML or Markdown

**Mood:** 😊
**Category:** Requests and ideas

### Summary

Currently, sessions can only be shared if they are quick chats. Sessions that are associated with a repository cannot be shared in the same way.

### Expected behavior

As a user, I would like to be able to share a repo-associated session as HTML or Markdown, so that I can export and distribute the conversation context and outcomes just like I can with quick chats.

### Current behavior

- Quick chat sessions have a share/export option.
- Repo-associated sessions do not expose a share or export option.
- There is no way to export a repo session to HTML or Markdown.

### Why this matters

Repo sessions often contain the most valuable context: code discussions, architecture decisions, and file-level reasoning. Being able to share that content as HTML or Markdown would make it easier to document decisions, hand off work, or publish summaries without manually copying fragments out of the chat.

### Suggested approach

Add a share/export action to repo-associated sessions that produces either:
- A self-contained HTML file with the full conversation rendered in the current theme, or
- A Markdown file with messages, code blocks, and any referenced files preserved.

---
| Field | Value |
| --- | --- |
| App version | 1.0.69-0 |
| Theme | GitHub |
| Path | /chat |

从 /chat 入口开始，将现有的 quick-chat 分享/导出流程与仓库关联的会话视图进行比较。完成的标准是仓库会话支持分享或导出为 HTML 或 Markdown，同时保留对话、代码块和引用的文件。

由索引模型根据 Issue 内容生成。