fix(coding-agent): prevent full re-renders during write tool streaming

Move line count from header to footer to avoid changing the first line
during streaming, which was triggering full screen re-renders in the
TUI's differential rendering logic.

packages/coding-agent/src/core/custom-tools/loader.ts

@@ -95,6 +95,7 @@ function createNoOpUIContext(): HookUIContext {
 		custom: async () => undefined as never,
 		setEditorText: () => {},
 		getEditorText: () => "",
+		editor: async () => undefined,
 		get theme() {
 			return theme;
 		},