feat(discord): add DISCORD_REPLY_TO_MODE setting (#6333)

Add configurable reply-reference behavior for Discord, matching the
existing Telegram (TELEGRAM_REPLY_TO_MODE) and Mattermost
(MATTERMOST_REPLY_MODE) implementations.

Modes:
- 'off': never reply-reference the original message
- 'first': reply-reference on first chunk only (default, current behavior)
- 'all': reply-reference on every chunk

Set DISCORD_REPLY_TO_MODE=off in .env to disable reply-to messages.

Changes:
- gateway/config.py: parse DISCORD_REPLY_TO_MODE env var
- gateway/platforms/discord.py: read reply_to_mode from config, respect
  it in send() — skip fetch_message entirely when 'off'
- hermes_cli/config.py: add to OPTIONAL_ENV_VARS for hermes setup
- 23 tests covering config, send behavior, env var override
- docs: discord.md env var table + environment-variables.md reference

Closes community request from Stuart on Discord.

gateway/config.py

@@ -712,6 +712,13 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
             name=os.getenv("DISCORD_HOME_CHANNEL_NAME", "Home"),
         )
     
+    # Reply threading mode for Discord (off/first/all)
+    discord_reply_mode = os.getenv("DISCORD_REPLY_TO_MODE", "").lower()
+    if discord_reply_mode in ("off", "first", "all"):
+        if Platform.DISCORD not in config.platforms:
+            config.platforms[Platform.DISCORD] = PlatformConfig()
+        config.platforms[Platform.DISCORD].reply_to_mode = discord_reply_mode
+    
     # WhatsApp (typically uses different auth mechanism)
     whatsapp_enabled = os.getenv("WHATSAPP_ENABLED", "").lower() in ("true", "1", "yes")
     if whatsapp_enabled: