chore: remove non-portable lefthook prepare script

[bytaesu]

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[greptile-apps]

Greptile Summary

This PR removes the non-portable package prepare script that automatically installed Lefthook Git hooks.

• Dependency installation no longer modifies the repository's Git hook configuration.
• The existing test, typecheck, and build scripts remain unchanged.

Confidence Score: 4/5

The PR appears safe to merge, though the removed setup path should be replaced with portable CI enforcement or documented manual installation for the checks it previously enabled.

New checkouts no longer install the configured pre-commit hooks, while the current CI workflow does not run the corresponding Biome and frozen-lockfile checks.

Files Needing Attention: package.json

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