[codex] Update release workflows for Node 24 actions

[adamziel]

What it does

Updates the GitHub Actions workflow dependencies that still emitted Node 20 deprecation warnings during the v0.1.48 publish run:

• actions/checkout@v6
• actions/cache@v5
• actions/upload-artifact@v7
• actions/download-artifact@v8
• softprops/action-gh-release@v3

It also refreshes docs/merge-reliability.md so the release-gate section points at the just-published v0.1.48 run and commit.

Rationale

The release succeeded, but GitHub warned that Node 20-based actions will be forced onto Node 24 soon. Keeping the release, verify, and CI workflows on current Node 24 action majors reduces future release risk before it turns into a broken publish path.

Implementation

The change is mechanical: replace the deprecated action majors across CI, release prepare, release verify, and release publish workflows. I verified each new action tag's action.yml advertises node24.

Testing instructions

Checked locally:

git diff --check
node scripts/release-validate.mjs --print-version
node -e "import('yaml').then(({parse})=>{for (const f of process.argv.slice(1)) parse(require('fs').readFileSync(f,'utf8'))})" .github/workflows/*.yml
grep -RInE "actions/(checkout|cache|upload-artifact|download-artifact)@v4|softprops/action-gh-release@v2" .github/workflows 2>/dev/null || true

Also verified the GitHub action refs via the GitHub API and confirmed each new tag uses node24.