refactor(default-mode): inject ultrawork via system prompt instead of visible text

When default_mode.ultrawork is enabled, inject the ultrawork behavioral
instructions into the system prompt (experimental.chat.system.transform)
instead of prepending them to the user's visible chat message.

- system-transform.ts: No-op handler now injects ultrawork message into
  output.system[] when defaultMode.ultrawork is active. Checks for existing
  <ultrawork-mode> tag to avoid re-injection after compaction.
- keyword-detector/hook.ts: Removes the visible text injection
  (output.parts[].text prepend) for default_mode.ultrawork path. Only
  shows the toast as the user-facing indicator.
- plugin-interface.ts: Wires getUltraworkMessage from keyword-detector
  into createSystemTransformHandler.

Result: Ultrawork mode activates silently via system prompt — the chat
stays clean, and only a toast shows 'Ultrawork Mode Active'.

src/plugin-interface.ts

@@ -7,6 +7,7 @@ import { createChatMessageHandler } from "./plugin/chat-message"
 import { createCommandExecuteBeforeHandler } from "./plugin/command-execute-before"
 import { createMessagesTransformHandler } from "./plugin/messages-transform"
 import { createSystemTransformHandler } from "./plugin/system-transform"
+import { getUltraworkMessage } from "./hooks/keyword-detector/ultrawork"
 import { createEventHandler } from "./plugin/event"
 import { createToolExecuteAfterHandler } from "./plugin/tool-execute-after"
 import { createToolExecuteBeforeHandler } from "./plugin/tool-execute-before"
@@ -58,7 +59,10 @@ export function createPluginInterface(args: {
       hooks,
     }),
 
-    "experimental.chat.system.transform": createSystemTransformHandler(),
+    "experimental.chat.system.transform": createSystemTransformHandler(
+      pluginConfig.default_mode,
+      getUltraworkMessage,
+    ),
 
     config: managers.configHandler,