Adds a /update command to Telegram, Discord, and other gateway platforms
that runs `hermes update` to pull the latest code, update dependencies,
sync skills, and restart the gateway.
Implementation:
- Spawns `hermes update` in a separate systemd scope (systemd-run --user
--scope) so the process survives the gateway restart that hermes update
triggers at the end. Falls back to nohup if systemd-run is unavailable.
- Writes a marker file (.update_pending.json) with the originating
platform and chat_id before spawning the update.
- On gateway startup, _send_update_notification() checks for the marker,
reads the captured update output, sends the results back to the user,
and cleans up.
Also:
- Registers /update as a Discord slash command
- Updates README.md, docs/messaging.md, docs/slash-commands.md
- Adds 18 tests covering handler, notification, and edge cases
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