Progresses stdlib-js/metr-issue-tracker#54.

Description

What is the purpose of this pull request?

This pull request:

• Switches Makefiles and pre-commit hook from legacy .eslintrc.* config to flat config autodiscovery
• Removes ESLINT_USE_FLAT_CONFIG=false, --ignore-path, --config, and per-target config variables from JS and TS eslint.mk.
• Removes per-directory tsconfig.json copy mechanism from TS eslint.mk (flat config's TS block handles this).
• Removes legacy config path variables from the pre-commit hook.
• ESLint now autodiscovers eslint.config.cjs and applies rules based on file patterns.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: migrate to ESLint v9 metr-issue-tracker#54

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

Stacked on #10972.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

If you answered "yes" above, how did you use AI assistance?

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

This PR was written primarily by Claude Code.

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@stdlib-js/reviewers