Opt-in flag that, after a release, creates or hard-resets a major
version branch ref (e.g. v1, v2) to the release commit and force-pushes
it to the remote.

This follows the GitHub Actions convention where consumers pin to a
major version ref (e.g. `uses: owner/action@v1`) and automatically
receive patch and minor updates without manual ref changes.

- New lib/major-branch.js: parses major from tag, runs git branch -f
  then git push --force origin <major-branch>
- --major-branch added to args (both CLIs, typedef, help text)
- preview.js shows the flag when enabled (hidden when false)

Unit tests for majorBranchName covering all tag formats (v-prefixed,
unprefixed, multi-digit, edge cases). Integration tests for
updateMajorBranch using a local bare repo as the remote, covering branch
creation, hard-reset to a newer commit, unparseable tag skip, and
un-prefixed tag handling.

Two correctness fixes from review:

- Use git rev-parse <tag>^{commit} to resolve the exact commit SHA from
  the tag, rather than passing opts.target_commitish which may be a
  branch name and could point to a newer commit if HEAD has moved.

- Switch from git branch -f to git update-ref refs/heads/<branch> so
  the major branch can be updated even when it is currently checked out
  (git branch -f refuses to move HEAD's branch).

Drop the commitish parameter from updateMajorBranch — the tag is now
the authoritative source. Update integration tests to create git tags
so git rev-parse resolves correctly.

Add the flag to both CLI help blocks and add a dedicated section
explaining when to use it, what it does step by step, and how to wire
it into package.json scripts.

Add --major-branch flag for GitHub Actions major version refs