ci(release): gate OSS desktop auto-update promotion (#5398)

## Summary

Separate OSS desktop artifact publication from fleet-wide auto-update
promotion.

- retain the exact generated updater manifest as `updater-manifest.json`
on each immutable `desktop-vX.Y.Z` release
- stop the tag-triggered build from mutating
`buzz-desktop-latest/latest.json`
- add a `main`-only manual promotion workflow with one global
concurrency group
- validate stable semver, release/tag commit identity, draft/prerelease
state, exact platform set, signatures, version-bound asset URLs, asset
existence, monotonicity, idempotent retries, and a final stale-state
check before writing
- document the operator flow and pin the split with focused contract
tests

## Safety behavior

Publishing a versioned GitHub release no longer exposes it through the
in-app updater. Operators can install and test those exact
signed/notarized artifacts, then manually run **Promote OSS Desktop
Auto-Update** with the stable version.

Promotion rejects downgrades. A same-version retry succeeds only when
the rolling and candidate manifests are byte-identical. The workflow
re-reads the current rolling version immediately before its only write
and records the actor, source tag commit, previous version, manifest
digest, and run URL.

## Verification

Verified at commit `39caf1603be06bb476905225ec55f7bbbe86b237`:

```text
scripts/test-oss-desktop-promotion.sh
OSS desktop promotion contract passed

scripts/test-release-ref-contract.sh
release ref contract passed

git diff --check origin/main...HEAD
(clean)
```

The repository pre-push hook also passed `branch-skew` for the exact
pushed head; package suites were correctly skipped because this change
only touches release workflows, scripts, and documentation.

Originating conversation: Buzz channel `separate-publish-step-release`,
thread
`8857ce8bbe928e891165eddcf06c666cf6eae16181c3f02a6d8c396d8a536026`.

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>

.github/workflows/ci.yml

@@ -78,6 +78,10 @@ jobs:
         run: scripts/test-release-ref-contract.sh
       - name: Desktop release candidate contract
         run: scripts/test-desktop-release-candidate.sh
+      - name: OSS desktop promotion contract
+        run: |
+          scripts/test-oss-desktop-promotion.sh
+          scripts/test-oss-desktop-promotion-behavior.sh
       - name: Mobile release contract
         run: |
           scripts/test-mobile-release-contract.sh