build: replace husky with lefthook for git hooks

[DivMode]

Summary

• Replace Husky v9 with Lefthook for git hook management
• Migrate conventional commit validation from .husky/commit-msg to lefthook.yml
• Migrate pre-push test runner from .husky/pre-push to lefthook.yml
• Single Go binary, faster startup, single-file config

Test plan

• Bad commit messages rejected by lefthook commit-msg hook
• Valid conventional commit messages accepted
• Pre-push hook runs full vitest suite (6/6 CF sites passed)