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/config.py

@@ -42,6 +42,7 @@ class Platform(Enum):
     SIGNAL = "signal"
     HOMEASSISTANT = "homeassistant"
     EMAIL = "email"
+    SMS = "sms"
     DINGTALK = "dingtalk"
 
 
@@ -232,6 +233,9 @@ class GatewayConfig:
             # Email uses extra dict for config (address + imap_host + smtp_host)
             elif platform == Platform.EMAIL and config.extra.get("address"):
                 connected.append(platform)
+            # SMS uses api_key (Twilio auth token) — SID checked via env
+            elif platform == Platform.SMS and os.getenv("TWILIO_ACCOUNT_SID"):
+                connected.append(platform)
         return connected
     
     def get_home_channel(self, platform: Platform) -> Optional[HomeChannel]:
@@ -564,6 +568,21 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
                 name=os.getenv("EMAIL_HOME_ADDRESS_NAME", "Home"),
             )
 
+    # SMS (Twilio)
+    twilio_sid = os.getenv("TWILIO_ACCOUNT_SID")
+    if twilio_sid:
+        if Platform.SMS not in config.platforms:
+            config.platforms[Platform.SMS] = PlatformConfig()
+        config.platforms[Platform.SMS].enabled = True
+        config.platforms[Platform.SMS].api_key = os.getenv("TWILIO_AUTH_TOKEN", "")
+        sms_home = os.getenv("SMS_HOME_CHANNEL")
+        if sms_home:
+            config.platforms[Platform.SMS].home_channel = HomeChannel(
+                platform=Platform.SMS,
+                chat_id=sms_home,
+                name=os.getenv("SMS_HOME_CHANNEL_NAME", "Home"),
+            )
+
     # Session settings
     idle_minutes = os.getenv("SESSION_IDLE_MINUTES")
     if idle_minutes: