docs: update AGENTS.md with script testing and env var guidelines

[maxandersen]

Updates AGENTS.md with lessons learned from #2459:

• Script consistency: note that behavior must be consistent across curl/wget/PowerShell, with a concrete example of tool-specific quirks (curl --retry-delay 0 vs wget --waitretry=0)
• Env var checklist: new env vars need defaults in all three scripts + documented in the reference table in installation.adoc
• Script testing: run real scripts from src/main/scripts/, not synthetic copies or extracted functions — use env var overrides to point at WireMock
• Conditional tests: concrete assumeTrue examples for shell availability