Enable file picker and markdown editor before submitting first prompt of session

### Feature summary

When opening a new session, user can see file picker in right rail and direclty edit, save, and commit files before first session in the prompt

### What problem are you trying to solve?

### TL;DR
As a user curating agent context in my repo, I often want to direclty edit files _before_ I engage an agent. For example, I may want to write a spec or product hypothesis, and then work with an agent to spar on that idea.

### Proposed solution

### Current state:
- Start session
- Right rail not availabel
- Sumbit first prompt of session
- Right rail option appears
- Click right rail, choose file picker, edit files

### Ideal state:
- Start session (local, worktree, etc.)
- RIght rail with file picker appears
- Choose file, edit, and then submit first prompt
- All changes should be able to be saved and commited before first prompt

### Workflow impact

### Why it's worth investing in

- Keeps user in GH app. The absence of a pre-session file picker and editor experience drives users to secondary editors (VS Code, Obsidian, etc.) outside of GH app. Introduces opportuntiy for incomplete session + creates context switching overhead for user.

### Installation context

_No response_

### Additional context

_No response_

从 issue 中描述的会话启动流程和右侧栏可见性条件入手；跟踪文件选择器、Markdown 编辑器以及保存和 commit 操作如何在首次 prompt 后变得可用。完成的标准是：新的本地或 Worktree 会话在首次 prompt 前显示右侧栏，并允许用户选择、编辑、保存和 commit 文件。

由索引模型根据 Issue 内容生成。