[feat] Add a "project group" or "folder" concept above projects #2745

Describe the feature

There is no way to group projects (repos) in the sidebar. The top-level sidebar is a flat scrollable list of projects (each individually collapsible, but with no mechanism to organize them into groups or folders)

For example, if you work across many repos (main app, sdks, docs, infra), they all sit at the same level. There's no way to tuck lower-priority projects like SDKs or docs repos into a collapsed group while keeping your primary repos prominent

Sections exist within projects to group workspaces (branches/worktrees), but nothing equivalent exists at the project level :(

Ideally there would be a "project group" or "project section" concept at the sidebar root level that can contain multiple projects.

Example:

Sidebar
├── Main App (project)
│   ├── main (workspace)
│   ├── feat/auth (workspace)
│   └── fix/header-bug (workspace)
├── Backend (project)
│   └── main (workspace)
├── SDKs (group
│   ├── python-sdk (project)
│   │   ├── main (workspace)
│   │   └── feat/streaming (workspace)
│   └── node-sdk (project)
│       └── main (workspace)
└── Infra (project)
    └── main (workspace)

Each group would be collapsible, renameable, and support reordering of projects into/out of groups similar to how workspace sections already work within a project