Review follow-ups for the branch-switch rollback:
- If the parked branch cannot be checked out again (for example, another
worktree holds it), restore its commit detached. Before this, the install
stayed on the broken update branch, and the recovery hint repeated the
failing command.
- Any `merge-base --is-ancestor` result other than "contained" keeps the
strict baseline. rc 128 used to fall back to the lenient one, which
contradicted its own comment.
- `_pull_updates` returns the baseline it verified, and
`_apply_pulled_update` checks against it. Before, that function
recomputed the baseline with a second rev-parse and merge-base against a
hardcoded `origin/<branch>`, which a fork push in between could move.
- Drop a `commit_count == -1` check that could never be false.
- Update the rollback description in `updating.md`.
- Tests share the fixture `git` helper, a repo-init helper and a `pull()`
wrapper instead of repeating those calls.
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