cursor-review: label-scoped concurrency lets a veto race an in-flight panel; workflows_ref duplicates the uses: SHA by hand

Two findings from the Cursor panel reviewing a thin caller (comfy-typescript-sdk#111), both properties of the shared pattern rather than of any one caller, so filing here where the fix can land once.

## 1. The documented caller concurrency group can't cancel across labels
The recommended caller sets:
```yaml
group: cursor-review-pr-${{ github.event.pull_request.number }}-${{ github.event.label.name }}
```
The label in the group is load-bearing (this workflow fires on every `labeled`/`unlabeled` event, and a shared per-PR group with `cancel-in-progress: true` would let ANY label add kill an in-flight panel). But it also means the two events that interact — `labeled: cursor-review` and the veto/unblock label — land in different groups and never cancel each other: applying `skip-cursor-review` while a panel is mid-flight runs a Gate that vetoes nothing already running, and toggling both within a panel's runtime can land two full reviews (the head-SHA dedupe is check-then-act, evaluated before either posts). Since the Gate owns the label semantics, the clean fix is probably Gate-side: on a veto event, cancel the PR's in-flight review runs via the API rather than relying on group collision.

## 2. `workflows_ref` is a hand-maintained duplicate of the `uses:` SHA
Every caller pins the reusable workflow by SHA and must copy the same SHA into `workflows_ref` so prompts/scripts load from the same commit — enforced only by a comment. A bump that updates one and not the other silently runs the workflow definition from one commit and its scripts from another. Options: default `workflows_ref` to the workflow's own ref where resolvable, or add a startup assertion that the two match and fail loudly.

Raised by: gpt-5.6-sol-max + claude-opus-5-thinking-max + kimi-k3-high panel legs on the caller PR.

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