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

Preflight Checklist

• [x] I have searched existing requests 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).