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.

hermes_cli/config.py

@@ -999,6 +999,13 @@ OPTIONAL_ENV_VARS = {
         "password": False,
         "category": "messaging",
     },
+    "DISCORD_REPLY_TO_MODE": {
+        "description": "Discord reply threading mode: 'off' (no reply references), 'first' (reply on first message only, default), 'all' (reply on every chunk)",
+        "prompt": "Discord reply mode (off/first/all)",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+    },
     "SLACK_BOT_TOKEN": {
         "description": "Slack bot token (xoxb-). Get from OAuth & Permissions after installing your app. "
                        "Required scopes: chat:write, app_mentions:read, channels:history, groups:history, "