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/app/host.go

@@ -832,6 +832,14 @@ func (h *toolHost) ScratchpadAppend(name, content string) error {
 	return err
 }
 
+func (h *toolHost) ScratchpadDelete(name string) error {
+	err := h.pads.Delete(name)
+	if err == nil && h.scratch != nil {
+		h.scratch.scratchpadsChanged()
+	}
+	return err
+}
+
 func (h *toolHost) WhoAmI(callerID string) mcp.WhoAmI {
 	w := mcp.WhoAmI{
 		ProcessID: callerID,
@@ -1091,7 +1099,7 @@ func availableToolsForRole(role mcp.CallerRole) []string {
 		"send_input", "send_message", "request_human_attention",
 		"timer_wait", "timer_set", "timer_fire_when_idle_any", "timer_fire_when_idle_all",
 		"timer_cancel", "timer_pause", "timer_resume", "timer_list",
-		"scratchpad_list", "scratchpad_read", "scratchpad_write", "scratchpad_append",
+		"scratchpad_list", "scratchpad_read", "scratchpad_write", "scratchpad_append", "scratchpad_delete",
 		"whoami", "help",
 	}
 	if role == mcp.RoleOrchestrator {
@@ -1146,8 +1154,8 @@ func helpFor(topic string) mcp.HelpResponse {
 	case "scratchpads":
 		return mcp.HelpResponse{
 			Topic:        "scratchpads",
-			Content:      "Project-scoped markdown files. Read returns content + revision; pass that back as expected_revision on write to get last-write-wins-with-detection. Append is unconditional.",
-			RelatedTools: []string{"scratchpad_list", "scratchpad_read", "scratchpad_write", "scratchpad_append"},
+			Content:      "Project-scoped markdown files. Read returns content + revision; pass that back as expected_revision on write to get last-write-wins-with-detection. Append is unconditional; delete removes a pad by name.",
+			RelatedTools: []string{"scratchpad_list", "scratchpad_read", "scratchpad_write", "scratchpad_append", "scratchpad_delete"},
 		}
 	case "timers":
 		return mcp.HelpResponse{

internal/app/scratchpad_delete_test.go

@@ -1,10 +1,12 @@
 package app
 
 import (
+	"errors"
 	"io"
 	"os"
 	"testing"
 
+	"github.com/hjbdev/patterm/internal/preset"
 	"github.com/hjbdev/patterm/internal/scratchpad"
 )
 
@@ -95,3 +97,41 @@ func TestDeletingLastFocusedScratchpadFocusesRunningChild(t *testing.T) {
 		t.Fatalf("focusedID = %q, want pid", st.focusedID)
 	}
 }
+
+type scratchpadChangeRecorder struct {
+	count int
+}
+
+func (r *scratchpadChangeRecorder) scratchpadsChanged() {
+	r.count++
+}
+
+func TestToolHostScratchpadDeleteRemovesPadAndRefreshes(t *testing.T) {
+	t.Setenv("XDG_DATA_HOME", t.TempDir())
+	pads, err := scratchpad.Open("scratchpad-delete-host-test")
+	if err != nil {
+		t.Fatalf("scratchpad.Open: %v", err)
+	}
+	if _, err := pads.Write("doomed.md", "content", ""); err != nil {
+		t.Fatalf("write doomed.md: %v", err)
+	}
+	recorder := &scratchpadChangeRecorder{}
+	host := newToolHost(nil, pads, nil, preset.Set{}, nil, 120, 40)
+	host.scratch = recorder
+
+	if err := host.ScratchpadDelete("doomed.md"); err != nil {
+		t.Fatalf("ScratchpadDelete: %v", err)
+	}
+	if recorder.count != 1 {
+		t.Fatalf("scratchpadsChanged calls = %d, want 1", recorder.count)
+	}
+	if _, _, err := pads.Read("doomed.md"); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("read deleted pad error = %v, want os.ErrNotExist", err)
+	}
+	if err := host.ScratchpadDelete("doomed.md"); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("delete missing error = %v, want os.ErrNotExist", err)
+	}
+	if recorder.count != 1 {
+		t.Fatalf("scratchpadsChanged calls after failed delete = %d, want 1", recorder.count)
+	}
+}