[CHORE] Plan migration to @actions/core v3 (ESM-only)

[c-vigo]

Chore Type

CI / Build change

Description

Create and approve a migration plan to safely adopt @actions/core@3 (ESM-only) in this action repository.
PR #62 shows the direct bump currently breaks packaging (ncc/CJS flow), so we need a deliberate migration path before enabling Dependabot major updates.

Acceptance Criteria

• Document current blocker from PR build(deps): bump @actions/core from 1.11.1 to 3.0.0 #62 (@actions/core@3 fails in current CommonJS packaging flow)
• Decide target runtime/module strategy (ESM build pipeline vs alternative packaging approach)
• Define required code/build/test changes and rollout sequence
• Define rollback/safety strategy if migration causes runtime regressions
• Update dependency policy to prevent accidental major bump merges before migration completion

Implementation Notes

• Current action runs on node20 with dist/index.js.
• Current source and bundled output are CommonJS-oriented.
• PR build(deps): bump @actions/core from 1.11.1 to 3.0.0 #62 should remain blocked until this migration plan (and then implementation) is complete.
• Include explicit validation steps for:
  • npm run build
  • npm run package
  • unit tests/integration tests
  • smoke test in a real workflow run

Related Issues

• Related to build(deps): bump @actions/core from 1.11.1 to 3.0.0 #62

Priority

High

Changelog Category

No changelog needed

Additional Context

Dependabot PR #62 (build(deps): bump @actions/core from 1.11.1 to 3.0.0) is currently incompatible with the repo’s packaging setup and should not be merged without this plan.