feat(mx): add Nextcloud Talk opencode bot pointing at halo.hoyer.tail:8000

Mirrors the existing nextcloud-claude-bot setup but invokes `opencode run`
against the local `halo-8000` provider/model. The bot listens on
127.0.0.1:8086, is exposed via the `/_opencode-bot/` location on
nc.hoyer.xyz, and uses `@Halo` as its mention trigger in group chats.

The opencode config (config/opencode/config.json) is installed into the
service's $HOME/.config/opencode/ on each start, so the bot picks up the
same provider definition the user uses interactively. The model map keys
are renamed to `halo-8000` / `halo-8001` so the canonical
`provider/model` reference works without an alias indirection.

systems/x86_64-linux/mx/default.nix

@@ -18,6 +18,7 @@
     ./network.nix
     ./nextcloud.nix
     ./nextcloud-claude-bot
+    ./nextcloud-opencode-bot
     ./nginx.nix
     ./ntfy.nix
     ./postgresql.nix