ci: enforce squash merges in Mergify

[gustavovalverde]

Motivation

Mergify did not specify a merge method, so enabling merge commits in the repository allowed queued PRs to create merge commits. Zebra's merge policy is squash-only.

Refs #10963.

Solution

Set merge_method: squash in the default queue rule so every queue uses the intended strategy explicitly.

Tests

• Parsed .github/mergify.yml as YAML.
• Verified the diff has no whitespace errors.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: OpenAI Codex for the configuration change and PR description

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.

[mergify]

Merge Protections

🟢 Merge protection satisfied — ready to merge.

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🟢 📃 Configuration Change Requirements

Mergify configuration change

• check-success = Configuration changed