Add scratchpad_delete MCP tool

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.

internal/mcp/protocol.go

@@ -358,6 +358,13 @@ func toolCatalog() []toolDescriptor {
 				"content": stringProp("Text to append."),
 			}, []string{"name", "content"}),
 		},
+		{
+			Name:        "scratchpad_delete",
+			Description: "Delete a scratchpad entry.",
+			InputSchema: objectSchema(map[string]any{
+				"name": stringProp("Scratchpad name."),
+			}, []string{"name"}),
+		},
 		{
 			Name:        "whoami",
 			Description: "Return the caller's identity, role, parent, project metadata, and available tools.",