fix: respect input_draft when updating existing release

[godfengliang]

Fixes #795

Problem

When updating an existing draft release to a prerelease, the draft field is hardcoded to existingRelease.draft (always true for a draft release). This means the release cannot be converted from draft to published prerelease even when draft input is explicitly set to false or omitted.

Root Cause

In src/github.ts line 582, the draft field in the updateRelease call uses existingRelease.draft directly, ignoring the user's input_draft setting. Compare this to the prerelease field on lines 569-570, which correctly respects config.input_prerelease.

Fix

Mirror the prerelease logic for draft:

Before:

draft: existingRelease.draft,

After:

draft: config.input_draft !== undefined ? config.input_draft : existingRelease.draft,

This ensures that when input_draft is explicitly provided, it overrides the existing value. When not provided, the existing draft status is preserved (backward compatible).

Testing

The user's reproduction case should now work:

1. Create a draft release with draft: true
2. In a subsequent workflow, set draft: false (or omit) and prerelease: true
3. The existing draft release is now correctly published as a prerelease