Mirrors the existing scratchpad_* tools end-to-end: catalog schema,
dispatch, ToolHost.ScratchpadDelete, and a host method that delegates to
scratchpad.Store.Delete and fires scratchpadsChanged() on success so the
sidebar refreshes. Missing-pad errors surface rather than being masked.
Resolves the [MCP SCRATCHPAD DELETE] TODO item.
Harry Bayliss
96f7c66d5f