Release gate: a merged standing PR carrying release:immediate is released twice #792

Problem

In standing-pr mode the release gate (packages/release/src/gate/evaluate-pr.ts) treats any merged PR that carries release:immediate as an immediate release. That includes the standing PR itself. If someone adds the label to the standing PR and merges it, releasekit release re-versions and publishes the same changes that standing-pr publish is publishing from the manifest, so the release is published twice or double-bumped.

#753 adds a workflow guard to the template, example and ci-setup snippet (github.event.pull_request.head.ref != 'release/next'). AGENTS.md says such guards hardcode defaults and the in-step checks are authoritative. Nothing in-step backs this one up, so the double publish still happens with:

• a renamed ci.standingPr.branch;
• a workflow that doesn't copy the guard, including one built on the reusable action.

Fix

• Have the gate refuse the immediate path when the merged PR's head branch is ci.standingPr.branch, with a reason such as "the standing release PR publishes from its manifest; release:immediate doesn't apply to it".
• The gate only has PR numbers today (findMergedPRsSinceLastRelease, then fetchPRLabels), so it needs the head ref. PullRequestDetails or AssociatedPullRequest can carry it.
• Test through releasekit gate and releasekit release with a merged standing PR that carries the label.