Clarify errs.Join nil behavior and update zap example

[spiegel-im-spiegel]

Summary

• document errs.Join nil-filtering behavior in the Join function comment
• add the same behavior note to README edge-case section
• add API stability note for errs.Join behavior to copilot instructions
• update zapobject example to include deferred file.Close errors in the return path using errs.Join

Validation

• task test
• task govulncheck

All checks passed locally.