Auto-assign spawned/child sessions to their parent's sidebar group

When a session spawns a child (a spawn_task chip, a continuation chip, a worktree agent), the child has no way to inherit the parent's sidebar group — confirmed by inspecting the full metadata schema returned by the session-management MCP tools (sessionId, title, cwd/originCwd, isArchived, isRunning, lastActivityAt, createdAt, model, isRemote, effort): there's no group field exposed anywhere, and no tool lets an agent set one.

Practical effect: if I organize sessions into a sidebar group for a project/thread, any chip or spawned session that comes out of that thread lands ungrouped, so the sidebar loses the association I built by hand.

Request: either (a) expose the parent's group ID to a spawned session so it can inherit it automatically, or (b) add a group field to session metadata that's settable via the MCP surface (alongside the existing set_session_title), so an agent could assign it explicitly.