docs(agents): clarify regression-focused tests

[christso]

Summary

• Clarifies that tests should primarily protect stable core contracts and regressions.
• Directs churny end-user behavior toward Astro docs instead of detailed brittle test matrices.
• Documents the docs-only git push --no-verify exception for changes that cannot affect runtime behavior.

Verification

• git diff --check
• Pushed with git push --no-verify because this is an AGENTS.md docs-only change.

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