chore: add Changesets release preparation

[ayeshLK]

Summary

• add exact-pinned @changesets/cli tooling compatible with the repository Node.js 20 development contract
• enable versioning for this private single-package Action without allowing Changesets to publish or tag it
• add npm run changeset for recording release intent and npm run release:version for version, changelog, and lockfile updates
• document contributor and maintainer workflows and update the pull request checklist
• include an empty changeset because this tooling-only change has no user-visible Action impact

Release preparation remains maintainer-controlled. The existing Release workflow still creates the draft, coordinates Marketplace publication, moves v0, and smoke-tests the published Action.

A bot-maintained version PR is intentionally not included: pull requests pushed with the repository GITHUB_TOKEN do not trigger the protected CI and Integration workflows. The repository-native script automates the error-prone versioning work without weakening branch protection or requiring a new privileged token.

Related issue

Follow-up release automation after #1, #10, and the published v0.1.2 release.

Verification

• I ran npm run check for code, test, or dependency changes.
• I ran git diff --check.
• I confirmed git diff --exit-code -- dist passes after building.
• Tests are unchanged; all 29 existing tests pass.
• The rebuilt dist/ bundle is unchanged.
• I synchronized contributor, maintainer, and pull request guidance.
• I added an empty changeset because this has no user-visible release impact.
• I ran changeset status successfully.
• I tested a synthetic patch release in an isolated copy: 0.1.2 became 0.1.3, CHANGELOG.md was generated, the changeset was consumed, and package-lock.json stayed synchronized.
• npm audit reports zero vulnerabilities.

Release impact

None. This introduces release tooling only and must not create a new Action release by itself.