Add setEditorText/getEditorText to hook UI context, improve custom() API

- Add setEditorText() and getEditorText() to HookUIContext for prompt generator pattern
- custom() now accepts async factories for fire-and-forget work
- Add CancellableLoader component to tui package
- Add BorderedLoader component for hooks with cancel UI
- Export HookAPI, HookContext, HookFactory from main package
- Update all examples to import from packages instead of relative paths
- Update hooks.md and custom-tools.md documentation

fixes #350

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

@@ -119,9 +119,25 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 			} as RpcHookUIRequest);
 		},
 
-		custom() {
+		async custom() {
 			// Custom UI not supported in RPC mode
-			return { close: () => {}, requestRender: () => {} };
+			return undefined as never;
+		},
+
+		setEditorText(text: string): void {
+			// Fire and forget - host can implement editor control
+			output({
+				type: "hook_ui_request",
+				id: crypto.randomUUID(),
+				method: "set_editor_text",
+				text,
+			} as RpcHookUIRequest);
+		},
+
+		getEditorText(): string {
+			// Synchronous method can't wait for RPC response
+			// Host should track editor state locally if needed
+			return "";
 		},
 	});
 

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

@@ -181,7 +181,8 @@ export type RpcHookUIRequest =
 			method: "notify";
 			message: string;
 			notifyType?: "info" | "warning" | "error";
-	  };
+	  }
+	| { type: "hook_ui_request"; id: string; method: "set_editor_text"; text: string };
 
 // ============================================================================
 // Hook UI Commands (stdin)