refactor: unify instance path resolution and preserve workspace override

nanobot/channels/discord.py

@@ -12,6 +12,7 @@ from loguru import logger
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.channels.base import BaseChannel
+from nanobot.config.paths import get_media_dir
 from nanobot.config.schema import DiscordConfig
 from nanobot.utils.helpers import split_message
 
@@ -289,7 +290,7 @@ class DiscordChannel(BaseChannel):
 
         content_parts = [content] if content else []
         media_paths: list[str] = []
-        media_dir = Path.home() / ".nanobot" / "media"
+        media_dir = get_media_dir("discord")
 
         for attachment in payload.get("attachments") or []:
             url = attachment.get("url")