[FEATURE] Archive a whole session group at once in the VS Code extension

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

Archiving works per session only. The sidebar offers "Archive session" / "Unarchive session" and an "Archived sessions" list, but there is nothing at group level.

When a work stream is finished — a released version, a closed ticket, a completed migration — its group stays in the sidebar in full, and clearing it means archiving every session in it one by one. Deleting the group is the only one-step alternative, but that discards the grouping itself, so the work cannot be brought back as a unit later.

Proposed Solution

Add "Archive group" to the group's context menu: archive the group together with the sessions it contains, remove it from the main sidebar list, and keep it restorable as a unit via "Unarchive group" — the group-level counterpart to the existing per-session archive.

Alternative Solutions

• Bulk-archiving the sessions inside a group via multi-select (requested in #67013 and #85704, both closed) would reduce the clicking but still leaves the now-empty group behind.
• Hiding empty groups (#95979, desktop) addresses the leftovers but not the archiving itself.

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

Environment

• Extension: Anthropic.claude-code 2.1.289 (linux-x64)
• Editors: VS Code and Cursor
• OS: Linux