Standardize action references on floating @v0 tag

[mmcky]

Summary

Every template, doc, and action README referenced quantecon/actions/...@v1 (116 occurrences), but no v1 tag exists — so copying any template produced a "tag not found" error. We're pre-1.0.0, and CONTRIBUTING.md reserves floating major tags (v1, v2) for after the 1.0.0 release, so @v0 is the correct floating reference during the 0.x phase.

This fixes review findings C1 (the @v1/no-tag mismatch) and N1 (the stale v0 tag).

Changes

• Switch all quantecon/actions/...@v1 → @v0 across templates, docs, and action READMEs. The unrelated action-download-artifact@v12 reference in PLAN.md was deliberately left untouched.
• README.md Versioning section: drop the false "@v1 - Latest stable v1.x.x release" claim (it contradicted CONTRIBUTING.md and reality); document @v0 (floating) / @v0.6.0 (pinned) / @main, with a warning that @v0 can move across a 0.x breaking change.
• CONTRIBUTING.md: add @v0 to the tag table and add a release step that force-moves the floating v0 tag to each new release.
• CHANGELOG.md: note under [Unreleased].
• Tidy-up: removed the scratch REVIEW-*.md technical-review notes from the repo.

⚠️ Required follow-up (maintainer action — not in this PR)

The v0 git tag is currently stale: it points at 6566f90 (v0.5.2-era, 2026-02-06), behind v0.6.0. A PR diff can't move a tag, so after merging, the floating tag must be force-moved for @v0 to resolve to current code:

git tag -f v0 v0.6.0      # or the latest release tag
git push origin v0 --force

This is an outward-facing tag operation that downstream lecture repos consume, so it's intentionally left for a maintainer to run (now documented in CONTRIBUTING.md).

Verification

• git grep 'quantecon/actions/...@v1' → no matches remain.
• action-download-artifact@v12 in PLAN.md → unchanged.
• 117 @v0 action references across the tree, internally consistent.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates QuantEcon lecture workflow templates and documentation to reference GitHub Actions using the floating @v0 tag (since no v1 tag exists and the project is still in 0.x), and documents the @v0 / pinned tag strategy in repo-level docs.

Changes:

• Replaced quantecon/actions/...@v1 references with @v0 across templates and documentation.
• Updated versioning guidance in README.md and release/tagging instructions in CONTRIBUTING.md to define how @v0 should be maintained.
• Added an [Unreleased] changelog entry describing the documentation/reference standardization.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
templates/publish.yml	Updates action references in the publish workflow template from @v1 to @v0.
templates/ci.yml	Updates action references in the CI/preview template from @v1 to @v0.
templates/cache.yml	Updates cache workflow template to use @v0.
setup-environment/README.md	Updates usage examples to reference actions via @v0.
restore-jupyter-cache/README.md	Updates usage examples to reference actions via @v0.
README.md	Rewrites the Versioning section to document @v0 (floating), a pinned @v0.x.y, and @main.
publish-gh-pages/README.md	Updates usage examples to reference publish-gh-pages via @v0.
preview-netlify/README.md	Updates usage examples to reference preview-netlify via @v0.
preview-cloudflare/README.md	Updates usage examples to reference preview-cloudflare via @v0.
docs/QUICK-REFERENCE.md	Updates quick-start snippets to use @v0.
docs/MIGRATION-GUIDE.md	Updates migration examples to use @v0.
docs/GPU-AMI-SETUP.md	Updates workflow example to use @v0.
docs/ARCHITECTURE.md	Updates architecture examples to use @v0.
CONTRIBUTING.md	Documents @v0 and adds release steps for moving the floating v0 tag.
CHANGELOG.md	Notes the docs/templates action reference standardization under [Unreleased].
build-lectures/README.md	Updates usage examples to reference build-lectures via @v0.
build-jupyter-cache/README.md	Updates usage examples to reference build-jupyter-cache via @v0.