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.

website/docs/reference/environment-variables.md

@@ -175,6 +175,7 @@ For cloud sandbox backends, persistence is filesystem-oriented. `TERMINAL_LIFETI
 | `DISCORD_REACTIONS` | Enable emoji reactions on messages during processing (default: `true`) |
 | `DISCORD_IGNORED_CHANNELS` | Comma-separated channel IDs where the bot never responds |
 | `DISCORD_NO_THREAD_CHANNELS` | Comma-separated channel IDs where bot responds without auto-threading |
+| `DISCORD_REPLY_TO_MODE` | Reply-reference behavior: `off`, `first` (default), or `all` |
 | `SLACK_BOT_TOKEN` | Slack bot token (`xoxb-...`) |
 | `SLACK_APP_TOKEN` | Slack app-level token (`xapp-...`, required for Socket Mode) |
 | `SLACK_ALLOWED_USERS` | Comma-separated Slack user IDs |