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/channel_directory.py

@@ -63,7 +63,7 @@ def build_channel_directory(adapters: Dict[Any, Any]) -> Dict[str, Any]:
             logger.warning("Channel directory: failed to build %s: %s", platform.value, e)
 
     # Telegram, WhatsApp & Signal can't enumerate chats -- pull from session history
-    for plat_name in ("telegram", "whatsapp", "signal", "email"):
+    for plat_name in ("telegram", "whatsapp", "signal", "email", "sms"):
         if plat_name not in platforms:
             platforms[plat_name] = _build_from_sessions(plat_name)