chore: ignore root-level Go binaries built from cmd/ - #3412

)

## What this PR does

The promote step guarded PR creation with `gh pr view <branch>`, which
resolves a pull request by head branch **in any state**. A closed one
satisfied the guard, so creation was skipped while the step still exited
0 — a promotion that reports success and leaves nothing to merge. Same
silent-skip shape as the enumeration bug fixed in #3404: the run is
green, the artifact is missing, and nothing says so.

`STABLE_BRANCH` is derived from the stable version
(`release-${version}`), not from the rc number, so every rc promoted to
that version shares it. Once any promote PR for a version has been
closed, both re-dispatch paths this workflow documents as supported are
wedged: re-dispatching the same rc, and promoting a newer rc to the same
version. That second one is named in the workflow's own header as the
reason a leftover draft is tolerated — the tolerance was implemented for
the release draft and missed for the PR.

Neither path recovers by reopening the old PR, because the step's own
`git checkout -B` plus force-push makes that PR's head unreachable, and
GitHub then refuses:

```
422 Validation Failed
  state cannot be changed. The release-1.6.0 branch was force-pushed or recreated.
```

The fix asks the question the guard meant to ask — is there an **open**
PR for this head — and creates one otherwise.

**Hit live.** Promoting `v1.6.0-rc.4` after #3397 had been closed: [run
29917852639](https://github.com/cozystack/cozystack/actions/runs/29917852639)
went green, logged `PR already open for release-1.6.0`, and opened no
PR. This currently blocks the v1.6.0 release.

**Verified in both directions** against live data, so the change is not
vacuous:

| head branch | PR state | old guard | new guard |
| --- | --- | --- | --- |
| `release-1.6.0` | #3397 closed | `TRUE` → skip (bug) | `FALSE` →
create ✅ |
| `chore/gitignore` | #3412 open | `TRUE` → skip | `TRUE` → skip ✅ |

`actionlint` exits 0 and `zizmor` reports no findings. The guard reads
`${STABLE_BRANCH}` as a shell variable from the step's `env:` block
rather than as a `${{ }}` expansion inside `run:`, so no
expression-injection surface is added.

### Screenshots

N/A — no UI change.

### Downstream repositories

Walked the trigger map in `docs/agents/contributing.md` against the
diff, which is one file: `.github/workflows/promote-rc.yaml`. The only
workflow-related trigger in the map is cozystack/ccp on "change
release-prep behaviour in `.github/workflows/tags.yaml`", which this
does not touch. This change also restores the documented behaviour of
`promote-rc.yaml` rather than altering its contract, so nothing
downstream sees a result different from what the docs already promise.

- [x] No downstream repository is affected by this change
- [ ] [cozystack/website](https://github.com/cozystack/website) -
follow-up:
- [ ]
[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
- follow-up:
- [ ]
[cozystack/ansible-cozystack](https://github.com/cozystack/ansible-cozystack)
- follow-up:
- [ ] [cozystack/ccp](https://github.com/cozystack/ccp) - follow-up:
- [ ] [cozystack/talm](https://github.com/cozystack/talm) - follow-up:
- [ ] [cozystack/cozyhr](https://github.com/cozystack/cozyhr) -
follow-up:
- [ ] [cozystack/cozy-proxy](https://github.com/cozystack/cozy-proxy) -
follow-up:
- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
- follow-up:
- [ ]
[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
- follow-up:
- [ ] [cozystack/examples](https://github.com/cozystack/examples) -
follow-up:

### Release note

```release-note
fix(release): promoting a release candidate now opens the promotion pull request even when an earlier promotion attempt for the same version was abandoned. Previously the workflow mistook a closed pull request for an open one, skipped creating a new one, and reported success with nothing left to merge.
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved release PR creation checks so closed or previously merged
pull requests no longer prevent new release PRs from being opened.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->