[WIP] Honor BUNDLE_JOBS env variable for bundle install jobs (fallback to 4 if unset)

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original description:

Update bundler.js so that the 'bundle install' command uses the number of jobs specified by the BUNDLE_JOBS environment variable (if set). If BUNDLE_JOBS is not set, default to using 4 jobs, as is done currently. This ensures users can control the concurrency of bundle install via environment variable, with backward compatibility to the default 4 jobs.

• The change should be implemented in the bundleInstall function where 'bundle install --jobs 4' is currently hardcoded (around line 209).
• The code should read the BUNDLE_JOBS environment variable and use its value if present, falling back to 4 otherwise.
• Add a comment explaining the logic for future maintainers.
• Update/add any related inline documentation if present.
• Add a brief note to the PR description describing the change and rationale.

Do not change any other behavior in the file.

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