feat(discord): add allowed_channels whitelist config

Add DISCORD_ALLOWED_CHANNELS (env var) / discord.allowed_channels (config.yaml)
support to restrict the bot to only respond in specified channels.

When set, messages from any channel NOT in the allowed list are silently
ignored — even if the bot is @mentioned. This provides a secure default-
deny posture vs the existing ignored_channels which is default-allow.

This is especially useful when bots in other channels may create new
channels dynamically (e.g., project bots) — a blacklist requires constant
maintenance while a whitelist is set-and-forget.

Follows the same config pattern as ignored_channels and free_response_channels:
- Env var: DISCORD_ALLOWED_CHANNELS (comma-separated channel IDs)
- Config: discord.allowed_channels (string or list of channel IDs)
- Env var takes precedence over config.yaml
- Empty/unset = no restriction (backward compatible)

Files changed:
- gateway/platforms/discord.py: check allowed_channels before ignored_channels
- gateway/config.py: map discord.allowed_channels → DISCORD_ALLOWED_CHANNELS
- hermes_cli/config.py: add allowed_channels to DEFAULT_CONFIG

hermes_cli/config.py

@@ -552,6 +552,7 @@ DEFAULT_CONFIG = {
     "discord": {
         "require_mention": True,       # Require @mention to respond in server channels
         "free_response_channels": "",  # Comma-separated channel IDs where bot responds without mention
+        "allowed_channels": "",        # If set, bot ONLY responds in these channel IDs (whitelist)
         "auto_thread": True,           # Auto-create threads on @mention in channels (like Slack)
         "reactions": True,             # Add 👀/✅/❌ reactions to messages during processing
     },