feat(ci): consolidate release workflows for npm OIDC trusted publishing (#3347)

Consolidate all release workflows into a single release.yml file to support
npm's Trusted Publishing (OIDC) which requires a single workflow file per package.

Modeled after React Router's approach:
https://github.com/remix-run/react-router/blob/a8f340129601f2efc848a6158eeff07a001cfa33/.github/workflows/release.yml

Changes:
- Create release.yml with all release jobs in one file
- Delete changesets.yml, next-release.yml, changesets-back-fix.yml
- Remove Slack announcement (workspace was deleted)

The new workflow has 4 jobs:
- release: Production releases via changesets (push to main)
- next-release: Snapshot releases with 'next' tag (push to main, skips [ci] release)
- backfix-release: Back-fix releases for calver branches (push to 2025-01, etc.)
- compile: Compiles templates to dist branch after production release

All jobs use NPM_TOKEN: '' to force OIDC authentication instead of token-based auth.

Author: kdaviduik