Support specifying github actions in mergit.yml

GitHub actions are represented in the GitHub API separately from the Commit statuses API. So we should support both.

```yml
# .mergit.yaml
expected:
statuses:
- ci/circleci: asdf
workflows:
"My workflow":
- build
- test
```
(transition: top-level `statuses` alias for `expected: { statuses: ... }`)

Graphql query:
```gql
query {
repository(...) {
object(...) {
... on Commit {
status {
# "ci/circleci: asdf"
contexts { context }
}
checkSuites(...) {
nodes {
workflowRun {
# "My workflow"
workflow { name }
}
checkRuns(...) {
# "build" + status
nodes { name, status }
}
}
}
}
}
}
}
```

Start with the .mergit.yaml example and the GraphQL query in the issue, then trace how expected statuses are read and checked against GitHub commit data. Support workflows alongside statuses, including workflow names and check-run names, while preserving the top-level statuses alias; done means both representations are evaluated correctly.

Written by the indexing model from the issue text.