feat(coding-agent): reimplement tree selector features

- Label editing with 'l' key
- Ctrl+O cycles through filters: default -> user-only -> labeled-only -> all
- Preserve cursor position when toggling filters
- Labels shown at front of node
- Normalize tabs/newlines in content
- extractContent limited to 200 chars for performance
- truncateToWidth on all rendered lines
- Iterative flattenTree to avoid stack overflow
- Linear chain optimization (no indent for single-child chains)

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

@@ -1647,6 +1647,10 @@ export class InteractiveMode {
 					done();
 					this.ui.requestRender();
 				},
+				(entryId, label) => {
+					this.sessionManager.appendLabelChange(entryId, label);
+					this.ui.requestRender();
+				},
 			);
 			return { component: selector, focus: selector };
 		});