Add intra-line diff highlighting for edit tool

- Extract diff rendering to dedicated diff.ts component
- Show word-level changes with inverse highlighting when a single line is modified
- Use diffWords for cleaner token grouping (includes adjacent whitespace)
- Only apply intra-line diff when exactly 1 removed and 1 added line (true modification)
- Multi-line changes show all removed then all added without incorrect pairing
- Add theme.inverse() method for inverse text styling

packages/coding-agent/src/modes/interactive/theme/theme.ts

@@ -351,6 +351,10 @@ export class Theme {
 		return chalk.underline(text);
 	}
 
+	inverse(text: string): string {
+		return chalk.inverse(text);
+	}
+
 	getFgAnsi(color: ThemeColor): string {
 		const ansi = this.fgColors.get(color);
 		if (!ansi) throw new Error(`Unknown theme color: ${color}`);