feat(core): enhance hook system with PreCompress replacement and Idle event

PreCompress hooks can now return newHistory to replace built-in compression.
Add Idle hook event that fires after configurable inactivity period.

- PreCompress: accept history in hook input, return newHistory to skip
  built-in summarization (CompressionStatus.HOOK_REPLACED)
- Idle: new HookEventName with fireIdleEvent, auto-activates when
  extensions register Idle hooks (default 300s, configurable via
  hooksConfig.idleTimeout)
- Hook can return hookSpecificOutput.prompt to auto-submit a message

packages/cli/src/config/settingsSchema.ts

@@ -2021,6 +2021,16 @@ const SETTINGS_SCHEMA = {
         description: 'Show visual indicators when hooks are executing.',
         showInDialog: true,
       },
+      idleTimeout: {
+        type: 'number',
+        label: 'Idle Timeout',
+        category: 'Advanced',
+        requiresRestart: false,
+        default: 0,
+        description:
+          'Time in seconds before the Idle hook fires when there is no user input. Set to 0 to disable.',
+        showInDialog: true,
+      },
     },
   },
 
@@ -2165,6 +2175,18 @@ const SETTINGS_SCHEMA = {
         ref: 'HookDefinitionArray',
         mergeStrategy: MergeStrategy.CONCAT,
       },
+      Idle: {
+        type: 'array',
+        label: 'Idle Hooks',
+        category: 'Advanced',
+        requiresRestart: false,
+        default: [],
+        description:
+          'Hooks that execute after a period of inactivity. Can trigger maintenance tasks like memory consolidation.',
+        showInDialog: false,
+        ref: 'HookDefinitionArray',
+        mergeStrategy: MergeStrategy.CONCAT,
+      },
     },
     additionalProperties: {
       type: 'array',

packages/cli/src/ui/hooks/useGeminiStream.ts

@@ -1874,6 +1874,64 @@ export const useGeminiStream = (
     storage,
   ]);
 
+  // Idle hook timer: fires after idleTimeout seconds of no activity.
+  // If idleTimeout is explicitly set, use it. Otherwise, if any Idle hooks
+  // are registered (e.g. by an extension), use a default of 300 seconds.
+  const DEFAULT_IDLE_TIMEOUT = 300;
+  const idleTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+  const configuredIdleTimeout = settings.merged.hooksConfig?.idleTimeout ?? 0;
+  useEffect(() => {
+    // Clear any existing timer
+    if (idleTimerRef.current) {
+      clearTimeout(idleTimerRef.current);
+      idleTimerRef.current = null;
+    }
+
+    if (streamingState !== StreamingState.Idle || !config.getEnableHooks()) {
+      return;
+    }
+
+    // Compute effective timeout: use configured value, or default if
+    // Idle hooks are registered (e.g. by an extension).
+    let idleTimeoutSeconds = configuredIdleTimeout;
+    if (idleTimeoutSeconds <= 0) {
+      const hookSystem = config.getHookSystem();
+      const hasIdleHooks = hookSystem
+        ?.getAllHooks()
+        .some((h) => h.eventName === 'Idle' && h.enabled);
+      idleTimeoutSeconds = hasIdleHooks ? DEFAULT_IDLE_TIMEOUT : 0;
+    }
+
+    if (idleTimeoutSeconds <= 0) {
+      return;
+    }
+
+    const startTime = Date.now();
+    idleTimerRef.current = setTimeout(async () => {
+      const hookSystem = config.getHookSystem();
+      if (!hookSystem) return;
+
+      const elapsed = Math.round((Date.now() - startTime) / 1000);
+      try {
+        const result = await hookSystem.fireIdleEvent(elapsed);
+        const prompt = result?.finalOutput?.hookSpecificOutput?.['prompt'];
+        if (typeof prompt === 'string' && prompt.trim()) {
+          // Auto-submit the prompt returned by the hook
+          void submitQuery(prompt);
+        }
+      } catch {
+        // Idle hook failures are non-fatal
+      }
+    }, idleTimeoutSeconds * 1000);
+
+    return () => {
+      if (idleTimerRef.current) {
+        clearTimeout(idleTimerRef.current);
+        idleTimerRef.current = null;
+      }
+    };
+  }, [streamingState, configuredIdleTimeout, config, submitQuery]);
+
   const lastOutputTime = Math.max(
     lastToolOutputTime,
     lastShellOutputTime,