feat: implement channel directory and message mirroring for cross-platform communication

- Introduced a new channel directory to cache reachable channels/contacts for messaging platforms, enhancing the send_message tool's ability to resolve human-friendly names to numeric IDs.
- Added functionality to mirror sent messages into the target's session transcript, providing context for cross-platform message delivery.
- Updated the send_message tool to support listing available targets and improved error handling for channel resolution.
- Enhanced the gateway to build and refresh the channel directory during startup and at regular intervals, ensuring up-to-date channel information.

cron/scheduler.py

@@ -119,6 +119,12 @@ def _deliver_result(job: dict, content: str) -> None:
         logger.error("Job '%s': delivery error: %s", job["id"], result["error"])
     else:
         logger.info("Job '%s': delivered to %s:%s", job["id"], platform_name, chat_id)
+        # Mirror the delivered content into the target's gateway session
+        try:
+            from gateway.mirror import mirror_to_session
+            mirror_to_session(platform_name, chat_id, content, source_label="cron")
+        except Exception:
+            pass
 
 
 def run_job(job: dict) -> tuple[bool, str, str, Optional[str]]: