fix(ci): repair preview-environment dispatch (use PAT, not GITHUB_TOKEN) (#20773)

## What
One-line token swap on the same-repo dispatch step in
[`preview-env-dispatch.yaml`](.github/workflows/preview-env-dispatch.yaml#L40):
`secrets.GITHUB_TOKEN` → `secrets.CI_PRIVILEGED_DISPATCH_TOKEN`.

## Why
Regression from [#20476](https://github.com/twentyhq/twenty/pull/20476)
("security: harden CI against supply-chain attacks"), merged 2026-05-12.
That PR replaced

```yaml
uses: peter-evans/repository-dispatch@v2
with:
  token: ${{ secrets.GITHUB_TOKEN }}
  ...
```

with a raw `gh api` call but kept `GITHUB_TOKEN`:

```yaml
env:
  GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
run: |
  gh api repos/"$REPOSITORY"/dispatches -f event_type=preview-environment ...
```

The auto-provisioned `GITHUB_TOKEN` can't fire `repository_dispatch` via
`gh api` even when the workflow declares `permissions: contents: write`.
The action used a different code path that worked; the CLI requires a
token with `repo` scope. So every dispatch from this workflow has
returned `403 Resource not accessible by integration` since that PR
merged — except for runs the `author_association` / `preview-app` label
gate skips entirely (which then show "success" because no jobs ran).

Recent failed example:
https://github.com/twentyhq/twenty/actions/runs/26162974597/job/76959379235?pr=20769

## The fix
`secrets.CI_PRIVILEGED_DISPATCH_TOKEN` already exists in repo secrets
and is **already used** by the immediately-following cross-repo dispatch
step in the same file. Using it for the same-repo dispatch too matches
the surrounding code and is consistent with the original hardening
intent (use a scoped PAT, not the auto-provisioned token).

## Test plan
- [ ] Merge this PR
- [ ] Next PR open / sync / reopen on a member's branch → check that
`Preview Environment Dispatch` succeeds (no 403)
- [ ] Confirm `Preview Environment Keep Alive` workflow gets triggered
(the downstream effect of the dispatch)
- [ ] Confirm the tunnel URL sticky comment lands on the PR

Discovered while testing an unrelated PR
([#20762](https://github.com/twentyhq/twenty/pull/20762)). Independent
fix.

.github/workflows/preview-env-dispatch.yaml

@@ -37,7 +37,7 @@ jobs:
     steps:
       - name: Trigger preview environment workflow
         env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_TOKEN: ${{ secrets.CI_PRIVILEGED_DISPATCH_TOKEN }}
           PR_NUMBER: ${{ github.event.pull_request.number }}
           PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
           REPOSITORY: ${{ github.repository }}