keep stable release pr open

[tarik02]

What Changed

• Always push release/stable and create/update the stable release PR, even when release package versions already match.
• Generate .github/release-pr/stable.json on the release branch so the PR has a real release marker diff.
• Replace the generated release PR body with stable-release context and commits since the previous stable tag.
• Let the release workflow recognize stable release PR merges through the release marker.

Why

After release infra fixes landed, package metadata still matched 2026.6.3000+tarik02, so the Release PR workflow reported unchanged metadata and skipped creating the follow-up release PR. We still need a persistent pending stable release PR until that release is merged.

UI Changes

n/a

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes