Support Windows runtimes and symbolic source refs

[sj26]

Why

Testing experimental Windows jobs with source-ref currently fails because the plugin only builds Linux and macOS runtimes. Testing a moving runtime branch also requires manually updating a full commit SHA.

What

Build Windows amd64 alongside Linux and macOS, and accept a commit, branch, or tag for source-ref. Resolve symbolic refs once and build every executable from that same commit. Annotated tags use their peeled commit; ambiguous branch/tag names fail rather than silently choosing one.

For a build with server-selected workflow context, the test configuration can omit workflow paths and runner overrides:

plugins:
  - github-actions#sj26/windows-source-ref:
      source-ref: sj26/gha-windows

The importer still runs on Linux amd64 or Darwin arm64. Experimental Windows jobs require compatible source (buildkite/buildkite-gha#488) and available runner mapping/queues. Stable release selection is unchanged.