chore: add lefthook pre-commit and pre-push hooks

[fagemx]

Summary

• Add lefthook.yml with cargo fmt --check on pre-commit and cargo clippy --workspace -- -D warnings on pre-push
• Document lefthook setup in CONTRIBUTING.md under a new "Git Hooks (optional)" section
• Local dev only, no CI impact

Closes #309

Test plan

• Install lefthook: npm install -g @evilmartians/lefthook && lefthook install
• Verify pre-commit runs fmt check: lefthook run pre-commit
• Verify pre-push runs clippy: lefthook run pre-push
• Verify CI passes (lefthook is not involved in CI)

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