Configure JetBrains Marketplace publishing

[redcatbear]

Situation

The project release workflow currently creates a GitHub release ZIP only. JetBrains Marketplace publishing is not configured in Gradle and there is no secret-backed Marketplace token path for publishPlugin. JetBrains also requires the first plugin publication to be uploaded manually before later Gradle uploads can be used.

Relevant docs: https://plugins.jetbrains.com/docs/intellij/publishing-plugin.html

Out-of-Scope

• Performing the first manual Marketplace upload.
• Creating or owning the JetBrains account, vendor profile, or Marketplace token.
• JetBrains Marketplace review outcome or review timing.
• Paid, freemium, or custom-channel publication.

Acceptance Criteria

1. Gradle Marketplace publishing is configured with a secret-backed publishing token.
2. The documented release path explains that publishPlugin is usable only after the first manual Marketplace upload exists.
3. The release workflow contains a Marketplace publication step or clearly documented disabled path that can be enabled after the first manual upload.
4. The Marketplace publication path uses the same version and signed artifact that passed the release verification gates.
5. The release documentation states which GitHub secret names are required and who must provision them outside the repository.