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.

gateway/status.py

@@ -0,0 +1,39 @@
+"""
+Gateway runtime status helpers.
+
+Provides PID-file based detection of whether the gateway daemon is running,
+used by send_message's check_fn to gate availability in the CLI.
+"""
+
+import os
+from pathlib import Path
+
+_PID_FILE = Path.home() / ".hermes" / "gateway.pid"
+
+
+def write_pid_file() -> None:
+    """Write the current process PID to the gateway PID file."""
+    _PID_FILE.parent.mkdir(parents=True, exist_ok=True)
+    _PID_FILE.write_text(str(os.getpid()))
+
+
+def remove_pid_file() -> None:
+    """Remove the gateway PID file if it exists."""
+    try:
+        _PID_FILE.unlink(missing_ok=True)
+    except Exception:
+        pass
+
+
+def is_gateway_running() -> bool:
+    """Check if the gateway daemon is currently running."""
+    if not _PID_FILE.exists():
+        return False
+    try:
+        pid = int(_PID_FILE.read_text().strip())
+        os.kill(pid, 0)  # signal 0 = existence check, no actual signal sent
+        return True
+    except (ValueError, ProcessLookupError, PermissionError):
+        # Stale PID file -- process is gone
+        remove_pid_file()
+        return False