docs(agents): forbid reinventing stdlib helpers and nil-passing

[cpcloud]

Summary

Two new rules in AGENTS.md under the "Architecture and code style" hard rules, both triggered by real incidents during recent eval-harness work:

• Check the stdlib before writing a helper: spend ten seconds looking for a stdlib equivalent before defining a tiny utility (boolStr, max, min, "convert primitive to string", "first non-zero value"). Go 1.21+ builtin min/max, strconv.FormatBool / FormatInt, slices.Contains, maps.Keys, cmp.Or — the stdlib covers most of these. A helper that reimplements stdlib adds surface area, hides the standard name, and tells reviewers "I didn't look."
• Check your nils: before passing a conceptual zero value (nil, "", 0) to a third-party function, read its godoc or grep for other call sites. Functions that query the terminal, open files, or dial the network commonly dereference their arguments. lipgloss.HasDarkBackground(os.Stdin, os.Stderr) — not (nil, nil). "I'll pass nil and see" is a red flag.

No code change.