fix: suppress duplicate cron sends to auto-delivery targets

Allow cron runs to keep using send_message for additional destinations, but
skip same-target sends when the scheduler will already auto-deliver the final
response there. Add prompt/tool guidance, docs, and regression coverage for
origin/home-channel resolution and thread-aware comparisons.

tests/agent/test_prompt_builder.py

@@ -455,6 +455,7 @@ class TestPromptBuilderConstants:
         assert "whatsapp" in PLATFORM_HINTS
         assert "telegram" in PLATFORM_HINTS
         assert "discord" in PLATFORM_HINTS
+        assert "cron" in PLATFORM_HINTS
         assert "cli" in PLATFORM_HINTS