feat(coding-agent): add ctx.ui.setEditorComponent() extension API

- Add setEditorComponent() to ctx.ui for custom editor components
- Add CustomEditor base class for extensions (handles app keybindings)
- Add keybindings parameter to ctx.ui.custom() factory (breaking change)
- Add modal-editor.ts example (vim-like modes)
- Add rainbow-editor.ts example (animated text highlighting)
- Update docs: extensions.md, tui.md Pattern 7
- Clean up terminal on TUI render errors

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

@@ -219,6 +219,10 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 			});
 		},
 
+		setEditorComponent(): void {
+			// Custom editor components not supported in RPC mode
+		},
+
 		get theme() {
 			return theme;
 		},