Repo level markdown files with architectural guidance not working for example… I’ve found that works quite well.

Same. AGENTS.md files and the like are quite effective. Especially if you’re reviewing the code and making the LLM help you update the markdown files when it makes a mistake to prevent the same type of mistake in the future. Having concrete examples of “good” vs. “bad” to illustrate each architectural rule goes a long way.

For any feature or bug fix that is “painting with the colors already in the tray”, it makes sense to let a LLM write the code. Humans will introduce new tech and new patterns out of boredom and turn the codebase into a big Frankenstein, but the LLM will just follow the architectural guidelines indefinitely.