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

gateway/platforms/discord.py

@@ -533,6 +533,16 @@ class DiscordAdapter(BasePlatformAdapter):
             except Exception as e:
                 logger.debug("Discord followup failed: %s", e)
 
+        @tree.command(name="update", description="Update Hermes Agent to the latest version")
+        async def slash_update(interaction: discord.Interaction):
+            await interaction.response.defer(ephemeral=True)
+            event = self._build_slash_event(interaction, "/update")
+            await self.handle_message(event)
+            try:
+                await interaction.followup.send("Update initiated~", ephemeral=True)
+            except Exception as e:
+                logger.debug("Discord followup failed: %s", e)
+
     def _build_slash_event(self, interaction: discord.Interaction, text: str) -> MessageEvent:
         """Build a MessageEvent from a Discord slash command interaction."""
         is_dm = isinstance(interaction.channel, discord.DMChannel)