[FEATURE] Nested session groups (groups within groups) in the desktop Code sidebar

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

The Code tab sidebar in the desktop app supports one level of session groups. Groups cannot contain other groups.

Once a project accumulates many sessions (per ticket, per PR, per experiment) a single flat group becomes a long undifferentiated list. Related sessions from the same workstream (for example one epic split across several tickets, or several repos that make up one product) cannot be kept together under a common parent, so navigating back to a session means scanning or searching every time.

### Proposed Solution

Allow a group to be nested inside another group in the Code sidebar, to at least one additional level (ideally arbitrary depth).

- Drag a group onto another group, or a "New subgroup" action in the group context menu.
- Collapse/expand state per subgroup, same as top-level groups today.
- "Start new session in this group" available on subgroups.
- Auto-derived per-project groups keep working as they do now and could sit inside a user-created parent group.

### Alternative Solutions

- Prefixing session titles with a path-style label (`auth/login`, `auth/mfa`) and relying on search. Works but is manual and does not collapse.
- Splitting into separate projects. Loses the "one product, several repos" view.

### Priority

Medium - Would be very helpful

### Feature Category

Other

### Use Case Example

1. One product made of a backend repo and a frontend repo, each with its own auto-derived project group.
2. Several tickets in flight per repo, each with its own session or worktree session.
3. With nested groups: parent group "Product" containing the two repo groups, each of which contains per-ticket subgroups.
4. Collapse everything not in progress and reopen the right session in one click.

### Additional Context

- Desktop app 1.49585.0, Claude Code 2.1.252, macOS.
- Docs describe the current single-level grouping: https://code.claude.com/docs/en/desktop.md ("Work in parallel with sessions").
- Related: #79156 (grouping is live-derived from git root), #91128 (project labels and grouping).

No source files or tests are named. Start by reading the desktop documentation section “Work in parallel with sessions” and inspecting the current single-level grouping behavior in the Code sidebar; done means groups can contain subgroups, subgroup collapse state and new-session actions work, and existing auto-derived project groups continue to work.

Written by the indexing model from the issue text.