Eighty percent of the harness, I can give you. A rules file, a way to load context, skills, gates, a sandbox, and a log at the end of every session. It is common sense, and it applies almost everywhere, because most of software development is the same everywhere.
The other twenty percent comes from your own sessions, and two things stop you from copying it. The first is the agent. Run the same prompt twice, in two sessions side by side, with the same model. Do you get exactly the same result? No. The agent is not deterministic: same input, different output.
The second is your company. One of you works in finance, one in a medical tech company, one in e-commerce; some of you use the agent directly, some through a cloud vendor, some have a different model altogether. You cannot copy and paste a strategy into all of that and expect it to work the same way.
A friend at another company sends you her whole harness: rules, gates, log format. It works beautifully there. Which part of it do you trust least on Monday?
The rules she wrote after her own mistakes. Rules that fit one agent's habits will meet different habits in yours. They are her twenty percent, and each one guards against a failure that happened in her company, with her model, on her data. Some will never fire for you. And the rules for your failures are not in it at all, because nobody has made those mistakes yet. The only way to earn them is the way I earned mine: experiment, observe, take notes, learn from the mistake. That is why harnessing starts with a sandbox, here meaning a practice copy of your system where mistakes cost nothing. Code climbs from the developer environment to QA to UAT to production as each one proves safe, and the agent climbs the same ladder. You would rather learn the military lessons from your scars, not from the military books. You will collect scars either way, and the sandbox is where you want to collect them.