(Not the author but) To your last question, I was able to replicate some of the benefits of my custom harness in Claude by just adding a custom MCP, startup hooks etc.

The issue I ran into is that I was fighting Claude's system prompt, which told it to do things which negated most of the benefits of my setup...

For context my startup hook was "grep def", lol. (Now working in js I have grep function, grep class, interface etc.). Gives it a little outline for context, saves time poking around blindly. My repos are small though, this is still a big blob if you have very large repos.