Add types: [checks_requested] to merge_group triggers

[nhorton]

Summary

• Adds explicit types: [checks_requested] to all merge_group triggers
• Updates README.md to document this best practice

Problem

Jobs were not starting when PRs entered the merge queue. The workflows have merge_group triggers with branches: [main], but without the explicit types parameter, GitHub may not properly trigger the workflows.

Solution

GitHub documentation recommends explicitly specifying types: [checks_requested] for merge_group triggers:

"By default, a workflow only runs when a merge_group event's activity type is checks_requested. However, it is recommended to explicitly specify the activity type to be specific about which events trigger the workflow, because the default may change if more activity types are added in the future."

This change adds the recommended types: [checks_requested] to all three workflows that use merge_group:

• validate.yml
• claude-code-test.yml
• cla.yml

Test plan

• Merge this PR via the merge queue
• Verify all jobs start and run correctly in the merge queue

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