feat(discord): add DISCORD_REPLY_TO_MODE setting

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/user-guide/messaging/discord.md

@@ -282,6 +282,7 @@ Discord behavior is controlled through two files: **`~/.hermes/.env`** for crede
 | `DISCORD_REACTIONS` | No | `true` | When `true`, the bot adds emoji reactions to messages during processing (👀 when starting, ✅ on success, ❌ on error). Set to `false` to disable reactions entirely. |
 | `DISCORD_IGNORED_CHANNELS` | No | — | Comma-separated channel IDs where the bot **never** responds, even when `@mentioned`. Takes priority over all other channel settings. |
 | `DISCORD_NO_THREAD_CHANNELS` | No | — | Comma-separated channel IDs where the bot responds directly in the channel instead of creating a thread. Only relevant when `DISCORD_AUTO_THREAD` is `true`. |
+| `DISCORD_REPLY_TO_MODE` | No | `"first"` | Controls reply-reference behavior: `"off"` — never reply to the original message, `"first"` — reply-reference on the first message chunk only (default), `"all"` — reply-reference on every chunk. |
 
 ### Config File (`config.yaml`)