[Feature Request] Session grouping by project/working directory in sidebar

Bug Description
Feature request: Session grouping by project/directory

Currently sessions appear in a flat list in the sidebar. When working across multiple projects, it becomes hard to find relevant sessions.

Codex (by OpenAI) already groups conversations by project, and it's a major quality-of-life improvement — you always know where you left off on a given codebase without scanning through an unsorted list of sessions. It makes context-switching between projects seamless.

Proposed: Group sessions by working directory (project) in the sidebar, similar to how IDEs organize recent files. Could be collapsible sections like:

▾ ~/Documents/TrainYourDragon (3 sessions)
▾ ~/Documents/OtherProject (2 sessions)

Alternatively, a tagging/labeling system would also work. The key need is quickly filtering to "show me all sessions for this project." This would be a significant quality-of-life improvement for anyone juggling multiple codebases.