feat(auto-install): migrate to ESM-only

[charliecreates]

Summary:

• Convert @rollup/plugin-auto-install to ESM-only.
• Update package metadata (type: module, exports, typings) and build output via tsc into dist.
• Migrate tests to Vitest.

Release notes:

• Version bumped to 4.0.0 (breaking: ESM-only).

Verification:

• pnpm -r build
• pnpm -r test
• pnpm -r lint

Closes #1935

[shellscape]

@CharlieHelps please do a thorough review of this PR to determine if it's ready to merge. Look at the diff, reviews, comments, and referenced issues/docs/links to build a complete understanding. Then respond with a comment that includes a list of MUST fix items that must be fixed before the the PR is merged, and a list of IMPROVEMENTS that shouldn't block the merge but may be quick improvements or things that could be addressed in a follow up PR. The MUST fix items should explain exactly what's wrong and what needs to change to resolve the problem. Both should be numbered lists and the IMPROVEMENTS list should continue counting up from the MUST list. It's OK if there are no IMPROVEMENTS or MUST fix items, you can just omit the lists.