Session Folders

### Feature summary

Add the ability to add folders to organize sessions into

### What problem are you trying to solve?

Sessions can be hard to track; it would be great if you could create a folder. One example of where this is useful is when multiple sessions are related to the same feature work.

### Proposed solution

_No response_

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by clarifying the intended folder workflow, including how sessions are created, moved, displayed, and persisted, then identify the relevant session-management surfaces. Done should include an agreed scope and tests for the folder behavior.

索引モデルが issue の本文から書いたものです。