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

gateway/config.py

@@ -581,6 +581,12 @@ def load_gateway_config() -> GatewayConfig:
                     if isinstance(ic, list):
                         ic = ",".join(str(v) for v in ic)
                     os.environ["DISCORD_IGNORED_CHANNELS"] = str(ic)
+                # allowed_channels: if set, bot ONLY responds in these channels (whitelist)
+                ac = discord_cfg.get("allowed_channels")
+                if ac is not None and not os.getenv("DISCORD_ALLOWED_CHANNELS"):
+                    if isinstance(ac, list):
+                        ac = ",".join(str(v) for v in ac)
+                    os.environ["DISCORD_ALLOWED_CHANNELS"] = str(ac)
                 # no_thread_channels: channels where bot responds directly without creating thread
                 ntc = discord_cfg.get("no_thread_channels")
                 if ntc is not None and not os.getenv("DISCORD_NO_THREAD_CHANNELS"):