feat(gateway): explain /voice usage when toggled bare (#39766)

A bare /voice silently toggled on/off with a one-line result, leaving
users with no idea what the modes mean or that Discord also supports
TTS-all and live voice-channel join/leave. Bare /voice now still
toggles but appends a usage explainer covering on/off/tts/status, with
the Discord voice-channel lines shown only on adapters that support
them.

Adds gateway.voice.help + gateway.voice.help_channels across all 16
locales (placeholders {toggle}/{channels}).

locales/en.yaml

@@ -358,6 +358,8 @@ gateway:
     label_off:             "Off (text only)"
     label_voice_only:      "On (voice reply to voice messages)"
     label_all:             "TTS (voice reply to all messages)"
+    help:                  "{toggle}\n\n**How /voice works**\n• `/voice on` — voice reply when you send a voice message\n• `/voice tts` — voice reply to *every* message\n• `/voice off` — back to text-only replies\n• `/voice status` — show the current mode\n• `/voice` (no argument) — quick toggle between on and off{channels}"
+    help_channels:         "\n\n**Live voice channels (Discord)**\n• Join a voice channel first, then `/voice channel` — I'll join, listen, and speak my replies\n• `/voice leave` — disconnect from the voice channel"
 
   yolo:
     disabled:                   "⚠️ YOLO mode **OFF** for this session — dangerous commands will require approval."