[Feature Request] Desktop sidebar: nested sub-groups within a project group

Feature request

Support nested sub-groups under a project group in the desktop app's session sidebar: a parent project heading with user-named child groups beneath it, each holding its own sessions.

Use case

A long-running research project accumulates many sessions that naturally cluster by sub-topic. Concretely: a geoscience project where one workstream ("Thermal Gradient Anchors") covers ~17 named wells, each of which gets multiple sessions over months. Today all of those land in one flat project group, ordered by recency, so finding "the sessions about well X" means scanning the whole list or maintaining a manual title-prefix convention.

Desired shape:

▾ Thermal Gradient Anchors
  ▸ (broad / cross-topic sessions)
  ▸ Bissa
  ▸ Anderville
  ▸ BN 1-9
  ...

Current state / workarounds

• The sidebar supports filter (status / project / environment) and group-by-project, but a group is a flat, recency-ordered list — no substructure, no pinning (#70104).
• Sub-folders of the same git repo collapse into a single block keyed at the repo root, so separate working directories inside one repo can't be used to fake sub-groups (#64354, closed stale).
• Workarounds in practice: session-title prefixes ("Bissa — …"), or umbrella directories of symlinks outside the repo to mint extra top-level groups. Both work but are manual and pollute either titles or the top-level sidebar.

Related open/closed asks (consolidating)

• #68171 — group sessions into folders/collections (open; closest sibling)
• #70104 — reorder/pin session groups (open)
• #64354 — nest same-repo sub-folders as separate blocks (closed, stale)
• #59016 — organize chats in folders by project (closed)
• #66877 — scope to a subfolder of a multi-folder workspace (closed; adjacent)
• #68262 — subfolders within a claude.ai Project (open; same ask on the claude.ai side)

Suggested minimal version

Even without full hierarchy: user-defined labels/tags on sessions, with the sidebar able to group by label within a project group, would cover the use case. Manual assignment is fine; no automation needed.