Allow selector-free Pipeline Trigger configuration

[sj26]

Why

GitHub Actions Pipeline Trigger builds receive trusted workflow selection from Buildkite, so they do not need an explicit workflow or workflows selector. When this private-preview, feature-flagged trigger type is enabled, the minimal pipeline is:

steps:
  - plugin: github-actions

The plugin schema currently requires a selector, preventing this shorthand and selector-free configurations that pin the runtime or configure runners and OIDC.

What

• Allow configurations with neither selector while continuing to reject both selectors and malformed known values.
• Preserve pass-through compatibility for supported runtime and behavioral options. The wrapper does not infer trigger context; buildkite-gha rejects selector-free configurations in ordinary builds.
• Keep explicit workflow selection as the primary README path and document the private-preview Pipeline Trigger mode with the trigger configuration.

The required Buildkite backend contract is deployed. A compatible buildkite-gha release must still be available before this plugin is released or promoted to latest.