Support uploads to draft releases

[zeitlinger]

Summary

Support uploading assets to existing draft releases.

Why

release-plz and similar immutable-release workflows create the GitHub release as a draft first, then upload assets, then publish the release after the matrix succeeds.

The action currently finishes with gh release upload <tag>, which looks up the release by tag and can fail for draft releases.

What changed

• replace the final gh release upload step with draft-aware upload logic
• resolve the release by matching tag_name from the releases API
• fetch the release upload_url and upload assets directly
• keep reruns idempotent by deleting an existing asset with the same name first
• document that existing draft releases are supported when using ref

Validation

• bash -n main.sh
• shellcheck main.sh

[taiki-e]

Could you tell me what the actual problem you're trying to solve is?

As said in #110, this isn't necessary with tag-based triggers or manual tag specification. Also, since release-based triggers aren't triggered by draft releases, the workflow won't run in the first place.

[zeitlinger]

Could you tell me what the actual problem you're trying to solve is?

As said in #110, this isn't necessary with tag-based triggers or manual tag specification. Also, since release-based triggers aren't triggered by draft releases, the workflow won't run in the first place.

I'll try to dig into this more - to be able to have a reproducer