doc: rewrite README for clarity

- Fix stale --compare-exact-versions flag (real flag is --major-only)
- Document GITHUB_TOKEN requirement and when it is/isn't needed
- Add uvx installation option
- Add analyze-orgs command
- Add --update shorthand example
- Improve examples and output samples

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fix:uv

fix:issue iterating over actions

feat:major versions update only support

fix:format length

feat:improve behavior

Update CHANGELOG.md