Fix exporter: correct --dir flag, handle full prompt sequence, find mail_* dir

- Replace --export-dir (doesn't exist) with --dir (correct v1.0.6 flag)
- Add handlers for the operation prompt (send 'B') and 'Do you wish to
  proceed?' (send 'Yes') which were previously unhandled
- After export, locate the actual mail_<timestamp> directory the CLI creates
  under <base>/<Email>/ and return it so callers get the right path
- Update prompt pattern for 'Username:' (CLI uses 'Username' not 'Email')
- _clean_export_dir now uses shutil.rmtree since the returned path is the
  mail_* subdir, not a flat file directory

https://claude.ai/code/session_01KjaNo9RXevw6x1DjJD8mj6

main.py

@@ -384,12 +384,13 @@ def _load_config() -> dict:
 
 
 def _clean_export_dir(export_dir: Path) -> None:
-    """Delete all files in the export directory after a sync cycle."""
-    for f in export_dir.iterdir():
-        try:
-            f.unlink()
-        except OSError as exc:
-            log.warning("Could not delete export file %s: %s", f.name, exc)
+    """Remove the mail_* export directory tree after a sync cycle."""
+    import shutil
+    try:
+        shutil.rmtree(export_dir)
+        log.debug("Removed export directory: %s", export_dir.name)
+    except OSError as exc:
+        log.warning("Could not remove export directory %s: %s", export_dir.name, exc)
 
 
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