[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 |

Start at the /chat entry point and compare the existing quick-chat share/export flow with repo-associated session views. Done means repo sessions expose sharing or export to HTML or Markdown while preserving the conversation, code blocks, and referenced files.

索引モデルが issue の本文から書いたものです。