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/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);