feat(02-05): Slack file_share extraction and channel-aware outbound routing
- Add gateway/channels/slack_media.py with is_file_share_event, media_type_from_mime,
build_slack_storage_key, build_attachment_from_slack_file, download_and_store_slack_file
- Add _send_response() helper to orchestrator/tasks.py for channel-aware dispatch
(Slack -> chat.update, WhatsApp -> send_whatsapp_message)
- Add send_whatsapp_message import to orchestrator/tasks.py for WhatsApp outbound
- Add boto3>=1.35.0 to gateway dependencies for MinIO S3 client
- Add 23 unit tests in test_slack_media.py (TDD)
Adolfo Delorenzo
9dd7c481a3