README: GitHub Actions example still says the action has no numbered release

[Maxerns]

Why this is a good first issue

The README GitHub Actions section still pins `getsotto/sotto-action@543d1af56ac81d1f1511d88c3d269106e8513a28` and says a numbered action release has not yet been published. `v1.1.0` has been published (2026-08-03). The example `sotto-version` is still `v0.4.0`; current CLI is `v0.5.0`.

Copy only in README.md. The action repo has matching issues (getsotto/sotto-action#10, #11).

What to do

In the GitHub Actions subsection of the README:

1. Use `getsotto/sotto-action@v1.1.0` in the example (you may keep a comment that SHA pinning is stronger).
2. Remove the sentence that a numbered action release has not been published.
3. Set `sotto-version: v0.5.0` in the example.
4. Keep the note that the action ref and `sotto-version` are independent, and that `sotto-version` must be an exact `vX.Y.Z`.

British English, no em dashes. Link this issue from the PR.

Out of scope

Do not change sotto-action itself. Do not edit install.sh.