feat: dispatch frontend PR preview environments to cloud (#9715)

## Summary

Add support for deploying full ephemeral preview environments from
frontend PRs. This is the frontend-side half — it sends `pr_number` and
`variant` (cpu/gpu) in the dispatch payload, and adds a cleanup dispatch
on PR close/unlabel.

### Changes

- **`cloud-dispatch-build.yaml`** — Add `pr_number` and `variant` to the
`frontend-asset-build` dispatch payload. Variant is derived from which
preview label triggered the event (`preview-cpu` → cpu, else gpu).

- **`cloud-dispatch-cleanup.yaml`** (new) — Fire-and-forget dispatch of
`frontend-preview-cleanup` to the cloud repo when a frontend PR is
closed or has its preview label removed. Enables synchronized teardown.

### Companion PR

Cloud-side: Comfy-Org/cloud (creates the `deploy-frontend-preview` job,
extends the reconciler)

### How it works

1. Label a frontend PR with `preview`, `preview-cpu`, or `preview-gpu`
2. Assets build and upload to GCS (existing flow)
3. Cloud deploys a full ephemeral env at `fe-pr-{N}.testenvs.comfy.org`
using all `:main` service tags
4. Subsequent pushes update the frontend SHA via AppSet upsert
5. On close/unlabel, cleanup dispatch triggers immediate teardown

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9715-feat-dispatch-frontend-PR-preview-environments-to-cloud-31f6d73d3650819da1b5ca5ce419e06e)
by [Unito](https://www.unito.io)

.github/workflows/cloud-dispatch-build.yaml

@@ -46,18 +46,35 @@ jobs:
           EVENT_NAME: ${{ github.event_name }}
           PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
           PR_HEAD_REF: ${{ github.event.pull_request.head.ref }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          ACTION: ${{ github.event.action }}
+          LABEL_NAME: ${{ github.event.label.name }}
+          PR_LABELS: ${{ toJson(github.event.pull_request.labels.*.name) }}
         run: |
           if [ "${EVENT_NAME}" = "pull_request" ]; then
             REF="${PR_HEAD_SHA}"
             BRANCH="${PR_HEAD_REF}"
+
+            # Derive variant from labels
+            VARIANT="gpu"
+            if [ "${ACTION}" = "labeled" ]; then
+              [ "${LABEL_NAME}" = "preview-cpu" ] && VARIANT="cpu"
+            else
+              # synchronize — check existing labels on the PR
+              echo "${PR_LABELS}" | grep -q '"preview-cpu"' && VARIANT="cpu"
+            fi
           else
             REF="${GITHUB_SHA}"
             BRANCH="${GITHUB_REF_NAME}"
+            PR_NUMBER=""
+            VARIANT=""
           fi
           payload="$(jq -nc \
             --arg ref "${REF}" \
             --arg branch "${BRANCH}" \
-            '{ref: $ref, branch: $branch}')"
+            --arg pr_number "${PR_NUMBER}" \
+            --arg variant "${VARIANT}" \
+            '{ref: $ref, branch: $branch, pr_number: $pr_number, variant: $variant}')"
           echo "json=${payload}" >> "${GITHUB_OUTPUT}"
 
       - name: Dispatch to cloud repo