chore: automate releases with changesets

[TimPietruskyRunPod]

Summary

• Adds @changesets/cli to automate version bumping, changelog generation, and GitHub Release creation
• New changesets.yml workflow: on merge to main, creates/updates a "Version Packages" PR; when that PR is merged, creates a GitHub Release which triggers the existing release.yml to build Docker images
• Simplifies release.yml by removing the tag push trigger and manual release creation step (changesets handles both now)
• Finalizes CHANGELOG.md at version 1.3.0 as the baseline for changesets
• Rewrites CONTRIBUTING.md with the new automated release workflow docs

How it works

1. Contributors run npx changeset in their PR branch and commit the .changeset/*.md file
2. On merge to main → changesets action creates/updates a "chore: version packages" PR (bumps package.json, updates CHANGELOG.md)
3. Version Packages PR is merged → changesets action creates a GitHub Release
4. GitHub Release triggers release.yml → builds and pushes Docker images

Test plan

• Merge this PR → changesets.yml runs, finds no pending changesets, checks for release 1.3.0 — should be a no-op (or create 1.3.0 release if it doesn't exist)
• Create a test PR with npx changeset (pick patch, write a description) → merge it
• Verify "chore: version packages" PR is auto-created with bumped version and updated CHANGELOG
• Merge the "version packages" PR → verify GitHub Release is created → verify release.yml triggers