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.

gateway/run.py

@@ -848,6 +848,7 @@ class GatewayRunner:
             os.getenv(v)
             for v in ("TELEGRAM_ALLOWED_USERS", "DISCORD_ALLOWED_USERS",
                        "WHATSAPP_ALLOWED_USERS", "SLACK_ALLOWED_USERS",
+                       "SMS_ALLOWED_USERS",
                        "GATEWAY_ALLOWED_USERS")
         )
         _allow_all = os.getenv("GATEWAY_ALLOW_ALL_USERS", "").lower() in ("true", "1", "yes")
@@ -1132,6 +1133,13 @@ class GatewayRunner:
                 return None
             return EmailAdapter(config)
 
+        elif platform == Platform.SMS:
+            from gateway.platforms.sms import SmsAdapter, check_sms_requirements
+            if not check_sms_requirements():
+                logger.warning("SMS: aiohttp not installed or TWILIO_ACCOUNT_SID/TWILIO_AUTH_TOKEN not set")
+                return None
+            return SmsAdapter(config)
+
         elif platform == Platform.DINGTALK:
             from gateway.platforms.dingtalk import DingTalkAdapter, check_dingtalk_requirements
             if not check_dingtalk_requirements():
@@ -1169,6 +1177,7 @@ class GatewayRunner:
             Platform.SLACK: "SLACK_ALLOWED_USERS",
             Platform.SIGNAL: "SIGNAL_ALLOWED_USERS",
             Platform.EMAIL: "EMAIL_ALLOWED_USERS",
+            Platform.SMS: "SMS_ALLOWED_USERS",
         }
         platform_allow_all_map = {
             Platform.TELEGRAM: "TELEGRAM_ALLOW_ALL_USERS",
@@ -1177,6 +1186,7 @@ class GatewayRunner:
             Platform.SLACK: "SLACK_ALLOW_ALL_USERS",
             Platform.SIGNAL: "SIGNAL_ALLOW_ALL_USERS",
             Platform.EMAIL: "EMAIL_ALLOW_ALL_USERS",
+            Platform.SMS: "SMS_ALLOW_ALL_USERS",
         }
 
         # Per-platform allow-all flag (e.g., DISCORD_ALLOW_ALL_USERS=true)