fix: skip review on merge commits

[vklimontovich]

The push: branches: [main] trigger fires on merge commits when PRs are merged.
The existing skip logic checks commits/{sha}/pulls but GitHub doesn't associate
merge commits with PRs via that API, so the check returns 0 and the review runs
redundantly.

Fix: check parent count first. Merge commits always have 2 parents — if
parents | length > 1, skip immediately before the PR association check.

[vklimontovich]

Looks good

[jitsu-code-review]

Summary: I reviewed the changes in this PR and found no actionable bugs, security issues, or correctness regressions. The merge-commit skip on push events is scoped and behaves consistently with the existing target-resolution flow.