feat: add /update slash command for gateway platforms

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

README.md

@@ -336,6 +336,8 @@ See [docs/messaging.md](docs/messaging.md) for advanced WhatsApp configuration.
 | `/sethome` | Set this chat as the home channel |
 | `/compress` | Manually compress conversation context |
 | `/usage` | Show token usage for this session |
+| `/reload-mcp` | Reload MCP servers from config |
+| `/update` | Update Hermes Agent to the latest version |
 | `/help` | Show available commands |
 | `/<skill-name>` | Invoke any installed skill (e.g., `/axolotl`, `/gif-search`) |