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/gemini.tsx

@@ -4,8 +4,13 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import type { Config } from '@qwen-code/qwen-code-core';
-import { InputFormat, logUserPrompt } from '@qwen-code/qwen-code-core';
+import {
+  InputFormat,
+  isDebugLoggingDegraded,
+  logUserPrompt,
+  Storage,
+  type Config,
+} from '@qwen-code/qwen-code-core';
 import { render } from 'ink';
 import dns from 'node:dns';
 import os from 'node:os';
@@ -419,6 +424,19 @@ export async function main() {
       return;
     }
 
+    // Print debug mode notice to stderr for non-interactive mode
+    if (config.getDebugMode()) {
+      console.error('Debug mode enabled');
+      console.error(
+        `Logging to: ${Storage.getDebugLogPath(config.getSessionId())}`,
+      );
+      if (isDebugLoggingDegraded()) {
+        console.error(
+          'Warning: Debug logging is degraded (write failures occurred)',
+        );
+      }
+    }
+
     // For non-stream-json mode, initialize config here
     if (inputFormat !== InputFormat.STREAM_JSON) {
       await config.initialize();