ci: version the private package under changesets v3 config defaults

[zimeg]

Summary

This pull request restores versioning of the private root package after the @changesets/cli v2 → v3 upgrade in #670, which is why the release PR (#668) closed and pending changesets are stranded (see #668, and the discussion on #670).

• build(deps): bump js-yaml and @changesets/cli #670 bumped @changesets/cli (2.31.0 → 3.0.0) and pulled in @changesets/config v4, which flips the default for versioning private packages.
• In v2, private packages were versioned automatically (no git tag). Per the changesets v3 migration guide, v3 disables both by default: "Changesets v3 now disables both behaviors by default as most projects don't version private packages."
• This repo's root package is private: true, so changeset version now finds nothing to bump and produces an empty diff. changesets/action then force-pushes main's tip onto changeset-release/main, closing the release PR with No commits between main and changeset-release/main.
• Fix: opt in explicitly with "privatePackages": { "version": true, "tag": false }. version: true restores the bump; tag: false keeps this repo's hand-rolled vX/vX.Y tagging in release.yml untouched (changesets never tagged here — this action is not published to npm).
• Also bumps the config $schema URL to @changesets/config@4.0.0 to match the resolved dependency.

Verified locally against the CI-resolved @changesets/cli@3.0.0 / @changesets/config@4.0.0: before the change changeset status shows no packages to bump; after it shows @slack/slack-github-action -> 4.1.0, changeset version consumes the pending changeset and bumps package.json/CHANGELOG.md, leaving a non-empty diff.

Testing

• After merge, watch the Release workflow run on main and confirm it recreates the chore: release PR (targeting 4.1.0) with a non-empty diff, rather than closing it.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1322807

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.86%. Comparing base (638050b) to head (1322807).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #672   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files           8        8           
  Lines         732      732           
=======================================
  Hits          731      731           
  Misses          1        1           

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[zimeg]

🚀 Confidence built in #673 so let's now merge this.