Refactor ESLint configuration checks in implement.md to address deprecation

[danwashusen]

Summary

• stop regenerating deprecated .eslintignore when a flat eslint.config.* is present
• keep .eslintignore guidance for legacy .eslintrc* configs only
• direct flat-config projects to rely on the config’s own ignores entries for required patterns

Testing

• n/a (docs change)

Fixes #1069

[danwashusen]

This feels a little odd to me because of the "If ignore file already exists" instructions, but it works and keeps the edit tight...

As noted in #1069, I feel this would be better as part of a tracked task created during planning. I'm happy to proceed down that path if you'd prefer @localden...