Version the GitHub Action so users can pin to stable releases

[pzfreo]

Currently users reference the action as uses: estampo/estampo/action@main, which means any change to action/action.yml on main immediately affects all consumers. This creates a stability risk — a refactor, flag change, or bug could break downstream workflows without warning.

Proposal

Publish versioned tags for the action so users can pin:

uses: estampo/estampo/action@v1      # major version tag (recommended)
uses: estampo/estampo/action@v1.2.0  # exact version

This follows the GitHub Actions versioning convention:

1. Tag action releases alongside (or as part of) estampo releases
2. Maintain a floating major version tag (v1) that points to the latest compatible release
3. Update README and AI prompt examples to use @v1 instead of @main

Why this matters

• Users on @main get unversioned changes with no opt-in
• Breaking changes to the action (metrics format, Docker flags, inputs) have no migration path
• The action is a public API surface — it needs the same stability guarantees as the Python package