feat(core,cli): add debug logging infrastructure (M0-M2)

Implement debug logfile foundation for routing internal diagnostics
to a per-session log file instead of polluting the terminal.

Core changes:
- Add DebugLogger interface and createDebugLogger() in debugLogger.ts
- Add Storage.getGlobalDebugDir() and getDebugLogPath() for log paths
- Add Config.getDebugLogger() method with session-scoped logger
- Track write failures via isDebugLoggingDegraded()

CLI changes:
- Add DebugModeNotification component for interactive startup notice
- Add non-interactive debug notice to stderr in gemini.tsx

Log files are written to ~/.qwen/debug/<sessionId>.txt with format:
2026-01-24T10:30:00.000Z [LEVEL] [TAG] message

Debug logging is always-on; debug mode only controls the startup notice.

packages/cli/src/ui/components/DebugModeNotification.tsx

@@ -0,0 +1,36 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { Box, Text } from 'ink';
+import { Storage, isDebugLoggingDegraded } from '@qwen-code/qwen-code-core';
+import { useConfig } from '../contexts/ConfigContext.js';
+import { theme } from '../semantic-colors.js';
+
+/**
+ * Displays debug mode status and log file path when debug mode is enabled.
+ */
+export const DebugModeNotification = () => {
+  const config = useConfig();
+
+  if (!config.getDebugMode()) {
+    return null;
+  }
+
+  const logPath = Storage.getDebugLogPath(config.getSessionId());
+  const isDegraded = isDebugLoggingDegraded();
+
+  return (
+    <Box paddingX={1} marginTop={1} flexDirection="column">
+      <Text color={theme.status.warning}>Debug mode enabled</Text>
+      <Text dimColor>Logging to: {logPath}</Text>
+      {isDegraded && (
+        <Text dimColor>
+          Warning: Debug logging is degraded (write failures occurred)
+        </Text>
+      )}
+    </Box>
+  );
+};

packages/cli/src/ui/components/Notifications.tsx

@@ -9,6 +9,7 @@ import { useAppContext } from '../contexts/AppContext.js';
 import { useUIState } from '../contexts/UIStateContext.js';
 import { theme } from '../semantic-colors.js';
 import { StreamingState } from '../types.js';
+import { DebugModeNotification } from './DebugModeNotification.js';
 import { UpdateNotification } from './UpdateNotification.js';
 
 export const Notifications = () => {
@@ -19,13 +20,10 @@ export const Notifications = () => {
   const showInitError =
     initError && streamingState !== StreamingState.Responding;
 
-  if (!showStartupWarnings && !showInitError && !updateInfo) {
-    return null;
-  }
-
   return (
     <>
       {updateInfo && <UpdateNotification message={updateInfo.message} />}
+      <DebugModeNotification />
       {showStartupWarnings && (
         <Box
           borderStyle="round"