fix(discord): skip obfuscated channels in directory and backfill enumeration

Discord's Channel Obfuscation change (announced Aug 12 2026, HTTP
enforcement Nov 16 2026) dispatches channels the bot lacks VIEW_CHANNEL
on with name "___hidden___", flag 1 << 17 (CHANNEL_OBFUSCATED), and
nulled fields. Without filtering, the channel directory lists phantom
"___hidden___" entries the agent can never post to, and wildcard
missed-message backfill wastes history reads on channels that always
403.

Adds is_discord_channel_obfuscated() to gateway/platforms/helpers.py
(checks the flag bit plus the sentinel name for discord.py builds that
don't expose the new flag) and applies it at both enumeration sites.

plugins/platforms/discord/adapter.py

@@ -2280,8 +2280,18 @@ class DiscordAdapter(DiscordMediaMixin, BasePlatformAdapter):
         seen: set[str] = set()
         candidate_channels = []
         if "*" in channel_ids:
+            from gateway.platforms.helpers import is_discord_channel_obfuscated
+
             for guild in getattr(self._client, "guilds", []) or []:
-                candidate_channels.extend(getattr(guild, "text_channels", []) or [])
+                # Skip obfuscated placeholders (bot lacks VIEW_CHANNEL;
+                # Discord dispatches them with name "___hidden___" +
+                # CHANNEL_OBFUSCATED flag as of the Aug 2026 privacy
+                # change). History reads on them always fail.
+                candidate_channels.extend(
+                    ch
+                    for ch in (getattr(guild, "text_channels", []) or [])
+                    if not is_discord_channel_obfuscated(ch)
+                )
         else:
             for channel_id in sorted(channel_ids):
                 channel = None