feat: add SMS (Twilio) platform adapter

Add SMS as a first-class messaging platform via the Twilio API.
Shares credentials with the existing telephony skill — same
TWILIO_ACCOUNT_SID, TWILIO_AUTH_TOKEN, TWILIO_PHONE_NUMBER env vars.

Adapter (gateway/platforms/sms.py):
- aiohttp webhook server for inbound (Twilio form-encoded POSTs)
- Twilio REST API with Basic auth for outbound
- Markdown stripping, smart chunking at 1600 chars
- Echo loop prevention, phone number redaction in logs

Integration (13 files):
- gateway config, run, channel_directory
- agent prompt_builder (SMS platform hint)
- cron scheduler, cronjob tools
- send_message_tool (_send_sms via Twilio API)
- toolsets (hermes-sms + hermes-gateway)
- gateway setup wizard, status display
- pyproject.toml (sms optional extra)
- 21 tests

Docs:
- website/docs/user-guide/messaging/sms.md (full setup guide)
- Updated messaging index (architecture, toolsets, security, links)
- Updated environment-variables.md reference

Inspired by PR #1575 (@sunsakis), rewritten for Twilio.

website/docs/user-guide/messaging/index.md

@@ -1,12 +1,12 @@
 ---
 sidebar_position: 1
 title: "Messaging Gateway"
-description: "Chat with Hermes from Telegram, Discord, Slack, WhatsApp, Signal, Email, Home Assistant, or your browser — architecture and setup overview"
+description: "Chat with Hermes from Telegram, Discord, Slack, WhatsApp, Signal, SMS, Email, Home Assistant, or your browser — architecture and setup overview"
 ---
 
 # Messaging Gateway
 
-Chat with Hermes from Telegram, Discord, Slack, WhatsApp, Signal, Email, Home Assistant, or your browser. The gateway is a single background process that connects to all your configured platforms, handles sessions, runs cron jobs, and delivers voice messages.
+Chat with Hermes from Telegram, Discord, Slack, WhatsApp, Signal, SMS, Email, Home Assistant, or your browser. The gateway is a single background process that connects to all your configured platforms, handles sessions, runs cron jobs, and delivers voice messages.
 
 For the full voice feature set — including CLI microphone mode, spoken replies in messaging, and Discord voice-channel conversations — see [Voice Mode](/docs/user-guide/features/voice-mode) and [Use Voice Mode with Hermes](/docs/guides/use-voice-mode-with-hermes).
 
@@ -21,6 +21,7 @@ flowchart TB
             wa[WhatsApp]
             sl[Slack]
             sig[Signal]
+            sms[SMS]
             em[Email]
             ha[Home Assistant]
         end
@@ -35,6 +36,7 @@ flowchart TB
     wa --> store
     sl --> store
     sig --> store
+    sms --> store
     em --> store
     ha --> store
     store --> agent
@@ -129,6 +131,7 @@ Configure per-platform overrides in `~/.hermes/gateway.json`:
 TELEGRAM_ALLOWED_USERS=123456789,987654321
 DISCORD_ALLOWED_USERS=123456789012345678
 SIGNAL_ALLOWED_USERS=+155****4567,+155****6543
+SMS_ALLOWED_USERS=+155****4567,+155****6543
 EMAIL_ALLOWED_USERS=trusted@example.com,colleague@work.com
 
 # Or allow
@@ -288,6 +291,7 @@ Each platform has its own toolset:
 | WhatsApp | `hermes-whatsapp` | Full tools including terminal |
 | Slack | `hermes-slack` | Full tools including terminal |
 | Signal | `hermes-signal` | Full tools including terminal |
+| SMS | `hermes-sms` | Full tools including terminal |
 | Email | `hermes-email` | Full tools including terminal |
 | Home Assistant | `hermes-homeassistant` | Full tools + HA device control (ha_list_entities, ha_get_state, ha_call_service, ha_list_services) |
 
@@ -298,5 +302,6 @@ Each platform has its own toolset:
 - [Slack Setup](slack.md)
 - [WhatsApp Setup](whatsapp.md)
 - [Signal Setup](signal.md)
+- [SMS Setup (Twilio)](sms.md)
 - [Email Setup](email.md)
 - [Home Assistant Integration](homeassistant.md)