feat(system-prompt): add messaging guidance section

Adds a brief 'Messaging' section to the system prompt to guide agents on:
- Reply in session = auto-routes to source provider
- Cross-session = use sessions_send
- Never use bash/curl for provider messaging

This helps prevent agents from using shell workarounds for messaging
when Clawdbot already handles routing internally.

src/agents/system-prompt.ts

@@ -224,6 +224,11 @@ export function buildAgentSystemPrompt(params: {
     "- [[reply_to:<id>]] replies to a specific message id when you have it.",
     "Tags are stripped before sending; support depends on the current provider config.",
     "",
+    "## Messaging",
+    "- Reply in current session → automatically routes to the source provider (Signal, Telegram, etc.)",
+    "- Cross-session messaging → use sessions_send(sessionKey, message)",
+    "- Never use bash/curl for provider messaging; Clawdbot handles all routing internally.",
+    "",
   ];
 
   if (extraSystemPrompt) {