Unable to start Agents with Shared Compute

**Describe the bug**
Unable to start Agents with Shared Compute. When I press start, it will start loading forever.
I don't think it is not doing anything as I observed increased request of the shared compute when i try to start it up.

The agent settings shows the following message that I'm not sure what it is about, nor can I search any documents about it:
`Buzz is waiting for the relay's member roster. Try again shortly; if this persists, check the relay's membership configuration.`

**Steps to reproduce**
1. Follow up on #5552
2. Find the Fizz card on the Agents screen.
3. Click the small play badge over its avatar

**Expected behavior**
Fizz will then start running and respond to messages

**Version and platform**
Find your version at the bottom of the Settings sidebar. Write "unknown" if you can't determine it.
- Buzz version: v0.5.9
- OS: Ubuntu 26.04 LTS

**Logs / additional context**
Paste any relevant logs, error messages, or screenshots here.
Harness log:
```

=== starting Fizz (6f757bba32777e3adca6d24b5f7c417c2b2c58d840c924e0c3f0d0ff5d6a144d) at 2026-08-11T06:56:38.563854899+00:00 ===
2026-08-11T06:56:38.590426Z INFO buzz_acp: buzz-acp: setup payload present, entering setup-listener mode
2026-08-11T06:56:38.590721Z INFO buzz_acp::setup_mode: buzz-acp entering setup mode agent=Fizz requirements=2
2026-08-11T06:56:38.660346Z INFO buzz_acp::setup_mode: setup-mode: connected and subscribed to membership notifications
2026-08-11T06:56:38.661433Z INFO buzz_acp: owner resolved from BUZZ_AUTH_TAG: f3a2d0a6fcd697d762e8ab6d3f79cf09f7440c115976cfa510a8f011a395c98a
2026-08-11T06:56:38.687428Z INFO buzz_acp::setup_mode: setup-mode: discovered 0 channel(s)
2026-08-11T06:56:38.687580Z WARN buzz_acp::setup_mode: setup-mode: no channel subscriptions resolved — nudge listener will sit idle
=== stopped Fizz (6f757bba32777e3adca6d24b5f7c417c2b2c58d840c924e0c3f0d0ff5d6a144d) at 2026-08-11T07:39:56.785238548+00:00 ===

=== starting Fizz (6f757bba32777e3adca6d24b5f7c417c2b2c58d840c924e0c3f0d0ff5d6a144d) at 2026-08-11T07:45:12.709857793+00:00 ===
2026-08-11T07:45:12.819783Z INFO buzz_acp: buzz-acp: setup payload present, entering setup-listener mode
2026-08-11T07:45:12.820636Z INFO buzz_acp::setup_mode: buzz-acp entering setup mode agent=Fizz requirements=2
2026-08-11T07:45:13.078958Z INFO buzz_acp::setup_mode: setup-mode: connected and subscribed to membership notifications
2026-08-11T07:45:13.082480Z INFO buzz_acp: owner resolved from BUZZ_AUTH_TAG: f3a2d0a6fcd697d762e8ab6d3f79cf09f7440c115976cfa510a8f011a395c98a
2026-08-11T07:45:13.153703Z INFO buzz_acp::setup_mode: setup-mode: discovered 0 channel(s)
2026-08-11T07:45:13.154312Z WARN buzz_acp::setup_mode: setup-mode: no channel subscriptions resolved — nudge listener will sit idle
=== stopped Fizz (6f757bba32777e3adca6d24b5f7c417c2b2c58d840c924e0c3f0d0ff5d6a144d) at 2026-08-11T07:45:25.162766043+00:00 ===
```

Start with the buzz_acp setup-mode path shown in the harness log, especially membership notifications and channel discovery; reproduce from the Fizz card and inspect why zero channels are resolved. Done means the shared-compute agent starts, responds to messages, and no longer reports that the nudge listener will sit idle.

Written by the indexing model from the issue text.