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.

agent/prompt_builder.py

@@ -161,6 +161,11 @@ PLATFORM_HINTS = {
         "You are a CLI AI Agent. Try not to use markdown but simple text "
         "renderable inside a terminal."
     ),
+    "sms": (
+        "You are communicating via SMS. Keep responses concise and use plain text "
+        "only — no markdown, no formatting. SMS messages are limited to ~1600 "
+        "characters, so be brief and direct."
+    ),
 }
 
 CONTEXT_FILE_MAX_CHARS = 20_000