Migrate to Sampo for Automated Release Management

This PR replaces our custom release workflows with Sampo, a modern tool for managing changelogs, versioning, and publishing to crates.io.

Changes:

• Replaced the manual label-based version bumping workflow with Sampo's changeset-based approach
• Added a new unified release workflow that:
  • Checks for pending changesets before releasing
  • Requires explicit approval in Slack before publishing
  • Handles version bumping, changelog updates, tagging, and publishing to crates.io
  • Provides Slack notifications throughout the release process
• Added Sampo configuration and initial changeset
• Updated documentation with new release instructions
• Removed the old RELEASES.md in favor of README instructions

How to Use:

1. When making changes, run sampo add to create a changeset
2. Create a PR with your changes and the changeset file
3. Add the release label when merging to trigger the release process
4. Approve the release in Slack when prompted

The workflow can also be triggered manually via the workflow dispatch trigger.