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;
 		},

packages/coding-agent/src/core/hooks/runner.ts

@@ -52,6 +52,7 @@ const noOpUIContext: HookUIContext = {
 	custom: async () => undefined as never,
 	setEditorText: () => {},
 	getEditorText: () => "",
+	editor: async () => undefined,
 	get theme() {
 		return theme;
 	},

packages/coding-agent/src/core/hooks/types.ts

@@ -119,6 +119,15 @@ export interface HookUIContext {
 	 */
 	getEditorText(): string;
 
+	/**
+	 * Show a multi-line editor for text editing.
+	 * Supports Ctrl+G to open external editor ($VISUAL or $EDITOR).
+	 * @param title - Title describing what is being edited
+	 * @param prefill - Optional initial text
+	 * @returns Edited text, or undefined if cancelled (Escape)
+	 */
+	editor(title: string, prefill?: string): Promise<string | undefined>;
+
 	/**
 	 * Get the current theme for styling text with ANSI codes.
 	 * Use theme.fg() and theme.bg() to style status text.