Add lefthook

[alvseven]

Summary

• Adds lefthook with pre-push hooks: lint-fix (biome), type-check, and tests
• Adds postinstall script to auto-install lefthook on bun install
• Gitignores local creation-test.ts scratch file

Test plan

• bun install installs lefthook hooks
• git push triggers pre-push: lint, type-check, tests all pass

[BernardoSM]

✅ Snyk checks have passed. No issues have been found so far.

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