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)

packages/orchestrator/orchestrator/tasks.py

@@ -44,6 +44,7 @@ import json
 import logging
 import uuid
 
+from gateway.channels.whatsapp import send_whatsapp_message
 from orchestrator.main import app
 from shared.models.message import KonstructMessage
 
@@ -524,6 +525,68 @@ def _extract_tool_name_from_confirmation(confirmation_message: str) -> str:
     return "unknown_tool"
 
 
+async def _send_response(
+    channel: str,
+    text: str,
+    extras: dict,
+) -> None:
+    """
+    Channel-aware outbound routing — dispatch a response to the correct channel.
+
+    Checks ``channel`` and routes to:
+      - ``slack``: calls ``_update_slack_placeholder`` to replace the "Thinking..." message
+      - ``whatsapp``: calls ``send_whatsapp_message`` via Meta Cloud API
+      - other channels: logs a warning and returns (no-op for unsupported channels)
+
+    Args:
+        channel: Channel name from KonstructMessage.channel (e.g. "slack", "whatsapp").
+        text:    Response text to send.
+        extras:  Channel-specific metadata dict.
+                 For Slack: ``bot_token``, ``channel_id``, ``placeholder_ts``
+                 For WhatsApp: ``phone_number_id``, ``bot_token`` (access_token), ``wa_id``
+    """
+    if channel == "slack":
+        bot_token: str = extras.get("bot_token", "") or ""
+        channel_id: str = extras.get("channel_id", "") or ""
+        placeholder_ts: str = extras.get("placeholder_ts", "") or ""
+
+        if not channel_id or not placeholder_ts:
+            logger.warning(
+                "_send_response: Slack channel missing channel_id or placeholder_ts in extras"
+            )
+            return
+
+        await _update_slack_placeholder(
+            bot_token=bot_token,
+            channel_id=channel_id,
+            placeholder_ts=placeholder_ts,
+            text=text,
+        )
+
+    elif channel == "whatsapp":
+        phone_number_id: str = extras.get("phone_number_id", "") or ""
+        access_token: str = extras.get("bot_token", "") or ""
+        wa_id: str = extras.get("wa_id", "") or ""
+
+        if not phone_number_id or not wa_id:
+            logger.warning(
+                "_send_response: WhatsApp channel missing phone_number_id or wa_id in extras"
+            )
+            return
+
+        await send_whatsapp_message(
+            phone_number_id=phone_number_id,
+            access_token=access_token,
+            recipient_wa_id=wa_id,
+            text=text,
+        )
+
+    else:
+        logger.warning(
+            "_send_response: unsupported channel=%r — response not delivered", channel
+        )
+
+
 async def _update_slack_placeholder(
     bot_token: str,
     channel_id: str,