start_gateway() now checks for an existing running instance via PID file
before starting. If another gateway is already running under the same
HERMES_HOME, it refuses to start with a clear error message directing the
user to 'hermes gateway restart' or 'hermes gateway stop'.
Also fixes gateway/status.py to respect the HERMES_HOME env var instead of
hardcoding ~/.hermes. This scopes the PID file per HERMES_HOME directory,
which lays the groundwork for future multi-profile support where distinct
HERMES_HOME directories can run concurrent gateway instances independently.
- 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.
