feat: add SMS (Telnyx) platform adapter

Implement SMS as a first-class messaging platform following
ADDING_A_PLATFORM.md checklist. All 16 integration points covered:

- gateway/platforms/sms.py: Core adapter with aiohttp webhook server,
  Telnyx REST API send, markdown stripping, 1600-char chunking,
  echo loop prevention, multi-number reply-from tracking
- gateway/config.py: Platform.SMS enum + env override block
- gateway/run.py: Adapter factory + auth maps (SMS_ALLOWED_USERS,
  SMS_ALLOW_ALL_USERS)
- toolsets.py: hermes-sms toolset + included in hermes-gateway
- cron/scheduler.py: SMS in platform_map for cron delivery
- tools/send_message_tool.py: SMS routing + _send_sms() standalone sender
- tools/cronjob_tools.py: 'sms' in deliver description
- gateway/channel_directory.py: SMS in session-based discovery
- agent/prompt_builder.py: SMS platform hint (plain text, concise)
- hermes_cli/status.py: SMS in platforms status display
- hermes_cli/gateway.py: SMS in setup wizard with Telnyx instructions
- pyproject.toml: sms optional dependency group (aiohttp>=3.9.0)
- tests/gateway/test_sms.py: Unit tests for config, format, truncate,
  echo prevention, requirements, toolset integration

Co-authored-by: sunsakis <teo@sunsakis.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

agent/prompt_builder.py

@@ -157,6 +157,12 @@ PLATFORM_HINTS = {
         "the scheduled destination, put it directly in your final response. Use "
         "send_message only for additional or different targets."
     ),
+    "sms": (
+        "You are communicating via SMS text messaging. Keep responses concise "
+        "and plain text only -- no markdown, no formatting. SMS has a 1600 "
+        "character limit per message (10 segments). Longer replies are split "
+        "across multiple messages. Be brief and direct."
+    ),
     "cli": (
         "You are a CLI AI Agent. Try not to use markdown but simple text "
         "renderable inside a terminal."