feat(coding-agent): replaced /copy subcommands with picker tree

- Added a fullscreen /copy tree of recent assistant messages with nested code blocks and a live preview pane.
- Removed the /copy last|code|all|cmd subcommands in favor of tree selection.
- Extracted copy-target assembly into a testable util.

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

@@ -2700,10 +2700,6 @@ export class InteractiveMode implements InteractiveModeContext {
 		return this.#commandController.handleShareCommand();
 	}
 
-	handleCopyCommand(sub?: string) {
-		return this.#commandController.handleCopyCommand(sub);
-	}
-
 	handleTodoCommand(args: string): Promise<void> {
 		return this.#todoCommandController.handleTodoCommand(args);
 	}
@@ -2936,6 +2932,10 @@ export class InteractiveMode implements InteractiveModeContext {
 		this.#selectorController.showUserMessageSelector();
 	}
 
+	showCopySelector(): void {
+		this.#selectorController.showCopySelector();
+	}
+
 	showTreeSelector(): void {
 		this.#selectorController.showTreeSelector();
 	}