Harden RepoContextBench v1 CI

[rodion-m]

What changed

• pins every GitHub Action to an immutable full commit SHA while retaining the upstream major version in comments
• rewrites artifact-hygiene checks into shellcheck-safe explicit conditions
• keeps the V1 dataset/result validation and public Pages deployment behavior unchanged

Why

This newly recreated public repository must remain the immutable V1 publication line while its CI remains reproducible and supply-chain safe.

Verification

• actionlint .github/workflows/*.yml
• V1 dataset/result JSON and JSONL validation
• public artifact hygiene checks
• frontend TypeScript, oxlint, Vitest (8 passed), and Vite build
• verified commit signature

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.