Use native flat Prettier config

[jeffwidman]

Summary

• replace FlatCompat.extends("prettier") with eslint-config-prettier/flat
• remove the compatibility-only path and URL setup
• remove the direct @eslint/eslintrc dependency

eslint-plugin-github still installs @eslint/eslintrc transitively for its own use.

Stack

• stacked on Adopt ESLint 10 recommended rules and ES2024 #1749
• merge Adopt ESLint 10 recommended rules and ES2024 #1749 first, then retarget this PR to main

Validation

• compared calculated rules and settings for source and test files
• npm clean-install
• npm run lint-check
• npm run format-check
• npx tsc --noEmit
• git diff --check

[Copilot]

Pull request overview

Replaces ESLint’s compatibility-based Prettier configuration with its native flat configuration.

Changes:

• Imports eslint-config-prettier/flat directly.
• Removes FlatCompat and path/URL setup.
• Removes the direct @eslint/eslintrc dependency while retaining its transitive installation.

Show a summary per file

File	Description
eslint.config.mjs	Uses the native flat Prettier configuration.
package.json	Removes the compatibility-only dependency.
package-lock.json	Updates the root dependency declaration.

Review details

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• Files reviewed: 2/3 changed files
• Comments generated: 0
• Review effort level: Balanced