New 'drone @backup share <file_path> [--public]': uploads a single file to Drive
(AIPass Backups/Shared), sets a read permission (default: restricted to the
authenticated user; --public: anyone-with-link), returns the webViewLink
(webContentLink fallback). Reuses upload_single_file + DriveClient; idempotent
via _find_existing_file; fail-loud on every path. 21 new tests, all Drive API
mocked (zero live calls). Existing commands untouched. DPLAN-0230 v1.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q3mZT61WsKVN3srCwVDBiW