docs: document the version release process

[fengmk2]

Adds a ### Releasing subsection under ## Development in the README, capturing the current release process:

• Releases are published as git tags only; there is no npm package, so package.json stays at 1.0.0.
• Consumers reference the moving major tag @v1 (or pin a SHA), so each release moves v1 forward.
• Steps: sync main and confirm dist/index.mjs is in sync, create the annotated v1.x.0 version tag (minor bump), move the v1 major tag to the same commit, then push the new tag and force-push v1.

Documents the process just used to cut v1.13.0.

[Copilot]

Pull request overview

Adds documentation to the README describing how this GitHub Action is released and versioned via git tags (including updating the moving v1 major tag), aligning the process with how consumers reference the action.

Changes:

• Documented that releases are git tags only (no npm package; package.json remains 1.0.0).
• Added step-by-step commands for building/validating dist/index.mjs, creating annotated tags, and pushing tags (including force-updating v1).

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