feat: add visual thinking level indicator via border colors

The horizontal borders around the input editor now change color based on
the current thinking level, providing immediate visual feedback:

- off: gray (default)
- minimal: dim blue
- low: blue
- medium: cyan
- high: magenta

The more thinking, the brighter/more vibrant the color.

Changes:
- Add public borderColor property to Editor component (packages/tui)
- Use borderColor instead of hardcoded chalk.gray for borders
- Add getThinkingBorderColor() helper in TuiRenderer
- Add updateEditorBorderColor() to apply color changes
- Update border color when:
  - Cycling with Tab key
  - Selecting via /thinking command
  - Restoring session with thinking level set

packages/coding-agent/src/tui/tui-renderer.ts

@@ -491,6 +491,9 @@ export class TuiRenderer {
 		// Update footer with loaded state
 		this.footer.updateState(state);
 
+		// Update editor border color based on current thinking level
+		this.updateEditorBorderColor();
+
 		// Render messages
 		for (let i = 0; i < state.messages.length; i++) {
 			const message = state.messages[i];
@@ -579,6 +582,31 @@ export class TuiRenderer {
 		}
 	}
 
+	private getThinkingBorderColor(level: ThinkingLevel): (str: string) => string {
+		// More thinking = more color (gray → dim colors → bright colors)
+		switch (level) {
+			case "off":
+				return chalk.gray;
+			case "minimal":
+				return chalk.dim.blue;
+			case "low":
+				return chalk.blue;
+			case "medium":
+				return chalk.cyan;
+			case "high":
+				return chalk.magenta;
+			default:
+				return chalk.gray;
+		}
+	}
+
+	private updateEditorBorderColor(): void {
+		const level = this.agent.state.thinkingLevel || "off";
+		const color = this.getThinkingBorderColor(level);
+		this.editor.borderColor = color;
+		this.ui.requestRender();
+	}
+
 	private cycleThinkingLevel(): boolean {
 		// Only cycle if model supports thinking
 		if (!this.agent.state.model?.reasoning) {
@@ -597,6 +625,9 @@ export class TuiRenderer {
 		// Save thinking level change to session
 		this.sessionManager.saveThinkingLevelChange(nextLevel);
 
+		// Update border color
+		this.updateEditorBorderColor();
+
 		// Show brief notification
 		this.chatContainer.addChild(new Spacer(1));
 		this.chatContainer.addChild(new Text(chalk.dim(`Thinking level: ${nextLevel}`), 1, 0));
@@ -635,6 +666,9 @@ export class TuiRenderer {
 				// Save thinking level change to session
 				this.sessionManager.saveThinkingLevelChange(level);
 
+				// Update border color
+				this.updateEditorBorderColor();
+
 				// Show confirmation message with proper spacing
 				this.chatContainer.addChild(new Spacer(1));
 				const confirmText = new Text(chalk.dim(`Thinking level: ${level}`), 1, 0);

packages/tui/src/components/editor.ts

@@ -28,6 +28,9 @@ export class Editor implements Component {
 
 	private config: TextEditorConfig = {};
 
+	// Border color (can be changed dynamically)
+	public borderColor: (str: string) => string = chalk.gray;
+
 	// Autocomplete support
 	private autocompleteProvider?: AutocompleteProvider;
 	private autocompleteList?: SelectList;
@@ -61,7 +64,7 @@ export class Editor implements Component {
 	}
 
 	render(width: number): string[] {
-		const horizontal = chalk.gray("─");
+		const horizontal = this.borderColor("─");
 
 		// Layout the text - use full width
 		const layoutLines = this.layoutText(width);