chore: pin CI workflows to v4 and publish block as stable

[AStaroverov]

Changes

• .github/workflows/build.yaml: switch actions/context/init and node-simple-pnpm.yaml from the temporary MILAB-6707_changeset-coverage-gate branch to the released @v4
• block/package.json: drop --unstable from prepublishOnly, so the block is published as a stable release
• normalize quoting in build.yaml

Changeset

An empty changeset is included — no version bump for the published block package is intended, this only affects CI configuration and publish flags.

Greptile Summary

This PR moves CI references from a temporary feature branch to the released v4 line and changes the block publication command from unstable to stable. It deliberately includes an empty changeset, so no package version is bumped.

• Reusable workflow — A GitHub Actions workflow invoked by another workflow. Both context/init and node-simple-pnpm.yaml now resolve through @v4 instead of @MILAB-6707_changeset-coverage-gate.
• Stable publication — Publishing a block without the development-channel marker. prepublishOnly no longer passes --unstable to block-tools publish.
• Changeset — Metadata used to record release intent and calculate version bumps. The added changeset has empty front matter and therefore records no package bump.
• Changeset coverage gate — CI validation requiring relevant package changes to be represented by a changeset. The workflow continues to pass require-package-path-bump: true.

Confidence Score: 5/5

The PR appears safe to merge because no concrete, changed-code-triggered failure was established.

The workflow references remain consistent with the repository’s existing v4 convention, while the possible reusable-workflow input mismatch and stable republishing collision depend on external contract or registry facts not established by the available evidence.

Important Files Changed

Filename	Overview
.github/workflows/build.yaml	Replaces temporary GitHub CI branch references with @v4 and normalizes YAML quoting; no locally verifiable workflow-contract defect was established.
block/package.json	Removes --unstable from prepublishOnly, making subsequent block publication stable; no concrete collision or overwrite behavior was established.
.changeset/plain-donkeys-shave.md	Adds an intentionally empty changeset so the CI-only and publication-flag changes do not bump a package version.

_{Reviews (1): Last reviewed commit: "chore: pin CI workflows to v4 and publis..." | Re-trigger Greptile}