style: be more brief with AGENTS.md to not overload context window

[jessesimpson36]

Which problem does the PR fix?

COPILOT_INSTRUCTIONS.md has never been followed for me on any AI agent I use whether it's github or opencode. This PR changes it to AGENTS.md so that it's agent-generic, and I also made it brief. COPILOT_INSTRUCTIONS was basically a "I am too lazy to make this myself so I'll have AI do it", and because I didn't know what to expect from it. But part of why it's not working well is because it's probably taking up too many tokens.

What's in this PR?

Checklist

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[jessesimpson36]

I also added this line:

Complex logic for CI pipelines (>20 lines) should be implemented as golang scripts inside the scripts directory and then called with github actions. Do not implement this in bash.

Hopefully this will get us away from using bash by default just because we have a large legacy of bash