chore: Configure merge_group trigger in main CI workflow

[rekmarks]

Configures the merge_group trigger and adds jobs to avoid rerunning the main workflow in the merge queue per MetaMask/snaps#3162 and MetaMask/snaps#3786.

This won't do anything until we've enabled the merge queue for the default branch in repository settings.

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Note

Introduces merge queue awareness and conditional execution to reduce redundant CI runs.

• Adds merge_group trigger and new check-skip-merge-queue job using MetaMask/github-tools/.../check-skip-merge-queue@v1 to detect up-to-date PRs
• Gates check-workflows (and downstream jobs) with needs/if to skip when in merge_group and skip-merge-queue is true
• Updates all-jobs-pass to depend on check-skip-merge-queue and succeed if either all-jobs-complete passed or skip-merge-queue is true

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