Add GitHub Actions concurrency to prevent CI queue stacking

[groupthinking]

Add concurrency groups to CI workflows so superseded runs cancel and the Actions queue drains instead of growing unbounded.

[linear-code]

GRV-406

[github-actions]

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[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• GRV-400
• ci: workflows have no concurrency groups, so obsolete runs stack instead of superseding #1519

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