feat(mcp): add delete_design tool

Wires DELETE /api/v1/designs/{id} into the MCP layer.
The backend already implements the endpoint and prevents
deleting the last remaining design.

- mcp/app/tools.py: Tool schema + _dispatch case
- mcp/tests/test_tools.py: 2 new tests (dispatch + schema)
30/30 tests passing.

mcp/tests/test_tools.py

@@ -262,6 +262,18 @@ def test_create_design_schema_requires_name():
     assert tool.inputSchema["required"] == ["name"]
 
 
+@pytest.mark.anyio
+async def test_delete_design(mock_backend):
+    await _dispatch("delete_design", {"design_id": "d-old"})
+    mock_backend.delete.assert_called_once_with("/api/v1/designs/d-old")
+
+
+def test_delete_design_schema_requires_design_id():
+    tool = next(t for t in TOOLS if t.name == "delete_design")
+    assert tool.inputSchema["required"] == ["design_id"]
+    assert "design_id" in tool.inputSchema["properties"]
+
+
 @pytest.mark.anyio
 async def test_unknown_tool():
     with pytest.raises(ValueError, match="Unknown tool"):