fix(discord): route DM role-auth opt-in through config.yaml (not env var)

Per repo policy, ~/.hermes/.env is for secrets only. Guild IDs are
behavioral configuration, not secrets. Replacing the
DISCORD_DM_ROLE_AUTH_GUILD env var from the original fix with
discord.dm_role_auth_guild in config.yaml.

- New module-level _read_dm_role_auth_guild() helper reads
  hermes_cli.config.read_raw_config()['discord']['dm_role_auth_guild'].
  Fails closed on any parse error (safe default = DM role-auth off).
- DEFAULT_CONFIG['discord'] gains dm_role_auth_guild: '' with a comment
  documenting the opt-in.
- Tests patch hermes_cli.config.read_raw_config directly (via the
  _set_dm_role_auth_guild helper) instead of setenv/delenv. 12 tests
  in test_discord_roles_dm_scope pass; no env var involvement.
- Docstring + module docstring + comments updated to reference
  discord.dm_role_auth_guild.
- E2E verified with real imports across 6 scenarios: unset, int,
  string, garbage, zero, and (crucially) env-var-only-no-config all
  return None except the valid int/string cases. Env var has zero
  effect — policy compliance confirmed.

hermes_cli/config.py

@@ -1108,6 +1108,12 @@ DEFAULT_CONFIG = {
         "auto_thread": True,           # Auto-create threads on @mention in channels (like Slack)
         "reactions": True,             # Add 👀/✅/❌ reactions to messages during processing
         "channel_prompts": {},         # Per-channel ephemeral system prompts (forum parents apply to child threads)
+        # Opt-in DM role-based auth (#12136). By default, DISCORD_ALLOWED_ROLES
+        # authorizes only guild messages in the role's own guild — DMs require
+        # DISCORD_ALLOWED_USERS. Set dm_role_auth_guild to a guild ID to also
+        # authorize DMs from members of that one trusted guild holding the
+        # allowed role. Unset / empty / 0 = secure default (DM role-auth off).
+        "dm_role_auth_guild": "",
         # discord / discord_admin tools: restrict which actions the agent may call.
         # Default (empty) = all actions allowed (subject to bot privileged intents).
         # Accepts comma-separated string ("list_guilds,list_channels,fetch_messages")