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)

pyproject.toml

@@ -3,7 +3,9 @@ name = "konstruct"
 version = "0.1.0"
 description = "AI workforce platform — channel-native AI employees for Slack, Teams, and more"
 requires-python = ">=3.12"
-dependencies = []
+dependencies = [
+    "boto3>=1.42.74",
+]
 
 [tool.uv.workspace]
 members = [