buzz-sprig container does not include Goose binary but UI allows Goose harness selection for k3s agents

## Problem

When creating a k3s agent via the Buzz desktop app with "Goose" as the agent harness, the agent deploys successfully and the pod shows as Running, but the agent never sends text replies — only emoji reactions.

There is no warning in the UI about this. The failure is silent.

## Root Cause

The `buzz-sprig` container image does not include the Goose binary.
`BUZZ_ACP_AGENT_COMMAND` is set to `goose` by the Goose harness, but `goose` does not exist inside the container.

Pod logs show:
```
agent failed to spawn: IO error: No such file or directory (os error 2)
```

Binaries available in buzz-sprig (`/usr/local/bin/`):
```
buzz, buzz-acp, buzz-agent, buzz-dev-mcp, sprig, sprig-entrypoint
```

Additionally, `BUZZ_ACP_AGENT_COMMAND` is a reserved env var and cannot be overridden via the desktop app environment variables UI — so there is no workaround from the UI side without recreating the agent.

## Steps to Reproduce

1. Create a k3s agent from Buzz desktop app
2. Select Agent harness: **Goose**
3. Set Run on: **kubernetes**
4. Use image: `ghcr.io/block/buzz-sprig@sha256:`
5. Deploy the agent
6. Send a message to the agent — it reacts with emoji but never sends a text reply

## Fix (workaround)

Use **Buzz Agent** harness instead of Goose — `buzz-agent` binary exists in the container and works correctly.

## Expected Behavior

One of the following:
1. Remove Goose as a selectable harness option when **Run on: kubernetes** is selected, OR
2. Include Goose in the `buzz-sprig` container image, OR
3. Show a clear error in the UI when the selected agent command binary is not found in the container

Start with the desktop app's harness-selection flow for Kubernetes and the buzz-sprig image configuration, then trace how BUZZ_ACP_AGENT_COMMAND is set. Verify the selected command exists in the container and that a missing command produces a clear UI outcome; done means Goose no longer deploys silently broken.

Written by the indexing model from the issue text.