feat(drive): add +download helper for downloading Drive files

.changeset/drive-download-helper.md

@@ -0,0 +1,22 @@
+---
+"@googleworkspace/cli": minor
+---
+
+Add `drive +download` helper for downloading Drive files to a local path
+
+The new `+download` command is a multi-step helper that:
+1. Fetches file metadata (name, MIME type) to determine how to download
+2. For Google Workspace native files (Docs, Sheets, Slides) uses `files.export`
+   with the caller-supplied `--mime-type` (e.g. `application/pdf`, `text/csv`)
+3. For all other files uses `files.get?alt=media`
+4. Writes the response bytes to a local path validated against path traversal
+
+This complements the existing `+upload` helper and follows all helper
+guidelines: it performs multi-step orchestration that the raw Discovery
+API cannot express as a single call.
+
+```
+gws drive +download --file FILE_ID
+gws drive +download --file FILE_ID --output report.pdf
+gws drive +download --file FILE_ID --mime-type application/pdf
+```