feat: delete multiple selected items at once with Delete key

[liam-machine]

Summary

• Multi-select on the canvas (shift-click or rubber-band drag), then Delete removes all selected items in one go — models, annotations, and standalone edges
• New removeModels batched message: one WorkspaceEdit, one save, one undo step for the entire batch
• Single batched VS Code prompt for "delete N model files?" replaces N back-to-back popups
• handleRemoveModel refactored to delegate into the batch handler (~80 lines of duplication removed)
• deleteKeyCode={null} on ReactFlow disables its built-in delete handler, which was racing our window-level handler and clearing selection

Test plan

• Shift-click 3+ models, press Delete — all disappear, single undo restores them
• Rubber-band-drag select (S key or shift-drag) over a region, Delete — same
• Single-node click + Delete still triggers the DetailPanel two-step confirm (unchanged)
• Multi-select that includes annotations and edges — all delete together
• In V5 domains, "delete N model files?" prompt appears once with correct singular/plural copy
• Physical (read-only) stage rejects multi-delete

🤖 Generated with Claude Code