Problem

The Control UI sessions sidebar only supports acting on one session at a time. Cleaning up after a busy day means archiving or deleting sessions one by one, each with its own context menu round-trip (and its own confirm dialog for delete). The Sessions settings page already has checkbox multi-select with batch delete, but the sidebar — where people actually live — has nothing.

Proposal

Multi-select in the sessions sidebar, matching familiar list UX:

• Cmd/Ctrl-click toggles a row in and out of the selection.
• Shift-click extends the selection from the anchor across the visible row order.
• Right-clicking (or opening the … menu on) a selected row shows a batch menu scoped to actions that apply to every selected session: Mark N as unread / Mark N as read, Move N to group, Archive N, Delete N….
• Right-clicking a row outside the selection retargets to that row (standard behavior).
• Batch delete asks for one confirmation and reuses the existing sessions.deleteMany batch path; preserved dirty worktrees surface as a single summary alert, same as the Sessions page.

Single-session menu behavior stays unchanged.

Evidence

• Sidebar menu host: ui/src/components/app-sidebar.ts (single SidebarSessionMenuState, per-session patchSession/deleteSession only).
• Menu component: ui/src/components/session-menu.ts (no batch mode).
• Batch RPC already exists: SessionCapability.deleteMany (ui/src/lib/sessions/index.ts), used by ui/src/pages/sessions/sessions-page.ts — the sidebar just never adopted it.