refactor: replace console.log/warn/error with file-based log() for silent logging

Replace all console output with shared logger to write to
/tmp/oh-my-opencode.log instead of stdout/stderr.

Files changed:
- index.ts: console.warn → log()
- hook-message-injector/injector.ts: console.warn → log()
- lsp/client.ts: console.error → log()
- ast-grep/downloader.ts: console.log/error → log()
- session-recovery/index.ts: console.error → log()
- comment-checker/downloader.ts: console.log/error → log()

CLI tools (install.ts, doctor, etc.) retain console output for UX.

src/index.ts

@@ -118,7 +118,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
     
     if (externalNotifier.detected && !forceEnable) {
       // External notification plugin detected - skip our notification to avoid conflicts
-      console.warn(getNotificationConflictWarning(externalNotifier.pluginName!));
+      log(getNotificationConflictWarning(externalNotifier.pluginName!));
       log("session-notification disabled due to external notifier conflict", {
         detected: externalNotifier.pluginName,
         allPlugins: externalNotifier.allPlugins,