Wire unit tests into CI (no test:unit script exists today)

[kriszyp]

Integration tests are running in CI on push to main / release branches and on PRs via integration-tests.yaml. The integration migration to @harperfast/integration-testing landed via HarperFast/harper-pro#103 and follow-ups.

What still needs work:

• package.json exposes no test / test:unit script — only test:integration*. The claude-issue-to-pr.yml workflow explicitly notes "There is no npm test / test:unit script — only npm run test:integration".
• unitTests/ and core/unitTests/ directories exist with test files (mocha-style based on .mocharc.json in core), but nothing in CI runs them.

Asks

• Add a test:unit script in package.json (and core/package.json if it doesn't already have one) that exercises the existing unitTests/ and core/unitTests/ trees.
• Add a unit-tests.yaml CI workflow (or extend integration-tests.yaml) to run npm run test:unit on push to main / release branches and on PRs.
• Drop the claude-issue-to-pr.yml comment about no unit-test script once it's wired up.

Acceptance criteria

• npm run test:unit runs all unit tests and exits non-zero on failure.
• CI runs unit tests on every PR and push to a release branch.
• README / CONTRIBUTING reflects the unit-test command.

Notes

• npm run build || true in the integration-tests workflow tolerates type errors today — keep that for the integration job until those are addressed separately, but the unit-test job shouldn't tolerate them.

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Tracked in Jira: CORE-3020
Jira fixVersion is v5.0 (already released / overdue) — retarget to V5.1.

🤖 Filed by Claude on behalf of Kris.

[kriszyp]

Addressed with #210