ops: add gitleaks pre-commit hook

Denis Morozovrequested to merge
feature/add-gitleaks-pre-commit into master

Summary

  • Add .pre-commit-config.yaml with gitleaks v8.30.0 hook for local secret detection
  • Document pre-commit setup in CONTRIBUTING.md

Pre-commit is the earliest detection point — faster feedback, lower blast radius. Complements CI-level gitleaks scanning.

Closes postgres-ai/infra#43

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