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/test/compaction-hooks.test.ts

@@ -113,6 +113,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 				custom: async () => undefined as never,
 				setEditorText: () => {},
 				getEditorText: () => "",
+				editor: async () => undefined,
 				get theme() {
 					return theme;
 				},