Upgrade to Node 24 with shared test workflow

[johnrb2]

Description of the change

Summary

• Upgrade Node.js test matrix to [20.x, 24.x] (drop EOL versions)
• Migrate test workflow to shared reusable workflow from activeprospect/github-action-workflows
• Add .nvmrc (Node 24) for local dev consistency
• Add engines field to package.json (>=20)

Part of the Node 24 upgrade effort (sc-107377).

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• UI Change Only
• Chore
• Configuration change
• Technical Debt
• Documentation

Related tickets

https://app.shortcut.com/activeprospect/story/107377

Checklists

Development and Testing

• Lint rules pass locally.
• The code changed/added as part of this pull request has been covered with tests, or this PR does not alter production code.
• All tests related to the changed code pass in development, or tests are not applicable.

Code Review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached.
• At least two engineers have been added as "Reviewers" on the pull request.
• Changes have been reviewed by at least two other engineers who did not write the code.
• This branch has been rebased off master to be current.

Tracking

• Issue from Shortcut/Jira has a link to this pull request.
• This PR has a link to the issue in Shortcut.

QA

• This branch has been deployed to staging and tested.

[josevsalina]

Node 24 upgrade — CI green, changes limited to expected scope (.nvmrc, test workflow, package.json engines). LGTM.