chore: replace semantic-release with changesets

[phoenix-server]

Summary

Replace the automated semantic-release pipeline with changesets for explicit, PR-level version management.

Changes

• Remove semantic-release and all @semantic-release/* plugins from package.json
• Delete .releaserc.json
• Initialize changesets with .changeset/config.json
• Add changeset-check CI job to checks.yml — fails PRs that are missing a changeset file
• Create changesets.yml workflow — creates Version PRs on main and publishes releases when merged
• Update README.md — document the new changeset workflow, remove semantic-release references

Why

Changesets give contributors and maintainers explicit control over version bumps and changelog entries at PR time, rather than inferring them from commit message format after the fact.

References

• https://github.com/changesets/changesets/blob/main/docs/detailed-explanation.md

[cameri]

This issue is not available for pick up.