️ Repo: Convert remaining `lib/cli/` files to ESM

Sub-issue of #5400 (Switch from CJS to ESM internally).

lib/cli/cli.mjs was converted in #5899. The following yargs command handler and helper files remain in CJS:

Files to convert

• lib/cli/collect-files.js: has a require.resolve()
• lib/cli/config.js: has a require.resolve()
• lib/cli/run.js
• lib/cli/run-helpers.js: has a require.resolve()
• lib/cli/watch-run.js: intentionally calls require() after a cache clear

lib/cli/index.js is an intentional CJS shim (module.exports = require("./cli.mjs")) and can remain CJS until the very end of the migration.

Notes

Per AGENTS.md CJS-to-ESM conventions:

• Prefer named exports over export default
• Rename each file from .js to .mjs
• Update consumers to use destructured require("./file.mjs")
• These files depend on lib/mocha.js — converting #5966 first is recommended