Support remote/server-hosted harness discovery (not just local PC)

## Problem

Buzz Desktop's "Agents" settings page (managed agents) only detects installed harnesses (Goose, Claude Code, Codex, Oh My Pi, etc.) by checking the local machine Desktop is running on — essentially probing PATH/binary presence locally.

This means the only supported way to run these agents is on the same PC that runs Buzz Desktop. Users who don't want to run an AI engine locally (for resource, security, or architecture reasons) and instead host their harnesses on a separate server have no way to make Desktop reflect that setup — even though the agent is fully functional as a bot via `buzz-acp` connecting to the relay, Desktop's local UI still shows it as "not installed."

## Current workaround

It's possible to get an agent running server-side today via `buzz-acp`:
- Compile `buzz-acp` on the server
- Generate a Nostr keypair for the agent (`buzz-admin generate-key`)
- Add it as a relay member (`buzz-admin add-member`)
- Publish a profile (`buzz-cli users set-profile`)
- Add it to a channel with `--role bot` (`buzz-cli channels add-member`)
- Run `buzz-acp` pointed at the harness binary and the relay

This works — the agent responds as a bot in channels — but it's entirely invisible/unmanaged from Desktop's "Agents" page, since that page only reflects local install status.

## Feature request

Add support for Desktop to discover/display harnesses that are hosted remotely (e.g. via a `buzz-acp` instance running on a server), instead of only checking the local machine. For example:
- A config option to point "Default harness" detection at a remote endpoint (SSH probe, or a small status API exposed by `buzz-acp` itself)
- Or, at minimum, a way to manually register a remote/server-hosted harness in the Agents page so its status doesn't show as "not installed" when it's actually running and healthy on a server

This would let self-hosters keep all agent compute on a server/homelab instead of the PC running Desktop, while still getting full visibility and management from the Desktop UI.

## Environment
- Self-hosted Buzz relay (Docker Compose)
- Harnesses tested: Oh My Pi (`omp acp`), via `buzz-acp`