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/app/tools.py

@@ -163,6 +163,13 @@ def _build_tools() -> list[Tool]:
                 "design_type": {"type": "string", "description": "Design type (default: network)."},
             },
         }),
+        Tool(name="delete_design", description="Delete a design (canvas) and all its nodes and edges. The last remaining design cannot be deleted.", inputSchema={
+            "type": "object",
+            "required": ["design_id"],
+            "properties": {
+                "design_id": {"type": "string", "description": "ID of the design to delete. Call list_designs to discover IDs."},
+            },
+        }),
     ]
 
 
@@ -272,4 +279,7 @@ async def _dispatch(name: str, args: dict) -> dict:
     if name == "create_design":
         return await backend.post("/api/v1/designs", args)
 
+    if name == "delete_design":
+        return await backend.delete(f"/api/v1/designs/{args['design_id']}")
+
     raise ValueError(f"Unknown tool: {name}")

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"):