Agents are referencing old instance with outdated harness

**Describe the bug**
Basically i setup my agents with one harness and then i stopped and edit the harness to another one and then restart the agent assuming the changes are going to take in place but on the channels they still reference the old instance with the harness that hit the limit.

**Steps to reproduce**
1. Go to a channel
2. Talk with an agent with some harness, example Codex
3. Hit the limit on codex
4. Go agents and change the agents harness to use another, like Claude
5. Try to talk with the agent
6. Still referencing the instance with the old harness (codex) and not able to respond

**Expected behavior**
Be able to change the harness smoothly and keep the conversation

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

**Logs / additional context**
I think the instance is not being updated correctly

Start by reproducing the channel conversation and harness change described in the issue, then trace how an agent instance is selected when the harness is edited and the agent restarts. No repository files or tests are named; done means the channel keeps the conversation while using the new harness instead of the old limited instance.

Written by the indexing model from the issue text.