fix(ci): drop --auto from nix-lock PR merge (#227)

GitHub rejects \`gh pr merge --auto\` when the PR has no required
status checks and no required reviews — the error is "Pull request
is in clean status (enablePullRequestAutoMerge)". Our ruleset
requires neither, so auto-merge has nothing to wait for. Switch
to a direct merge instead. If we ever add required checks, --auto
can come back.

Validated against the v1.43.0 test release: PR #226 was created
by the workflow but the auto-merge call failed; merging it
manually with the same flags minus --auto succeeded immediately.

Co-authored-by: Psychotoxical <dev@psysonic.app>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/release.yml

@@ -314,4 +314,4 @@ jobs:
             --head "$BRANCH" \
             --title "chore(nix): refresh lock + npmDepsHash for v${VERSION}" \
             --body "Auto-generated after the v${VERSION} release: refreshes \`flake.lock\` and \`nix/upstream-sources.json\` so the Cachix substituter resolves the latest pin."
-          gh pr merge "$BRANCH" --squash --auto --delete-branch
+          gh pr merge "$BRANCH" --squash --delete-branch