feat(gateway): thread-aware free-response routing for Discord

- Forum parent channel IDs now match free-response list (add a forum
  channel ID and all its threads respond without mention)
- Better thread chat names: 'Guild / forum / thread' for forum threads
- Add discord.require_mention and discord.free_response_channels to
  config.yaml (bridged to env vars, env vars still override)
- Keep require_mention defaulting to true (safe for shared servers)

Cherry-picked from PR #867 by insecurejezza with default fix and
config.yaml integration.

Co-authored-by: insecurejezza <insecurejezza@users.noreply.github.com>

hermes_cli/config.py

@@ -208,6 +208,12 @@ DEFAULT_CONFIG = {
     # Empty string means use server-local time.
     "timezone": "",
 
+    # Discord platform settings (gateway mode)
+    "discord": {
+        "require_mention": True,       # Require @mention to respond in server channels
+        "free_response_channels": "",  # Comma-separated channel IDs where bot responds without mention
+    },
+
     # Permanently allowed dangerous command patterns (added via "always" approval)
     "command_allowlist": [],
     # User-defined quick commands that bypass the agent loop (type: exec only)