ci: add lefthook PR title check

[longlho]

Summary

• add lefthook with a pre-commit PR title validator
• add a GitHub Actions workflow that runs lefthook on changed files
• document the hook and PR title convention in AGENTS.md
• ignore package-local node_modules output created by pnpm installs

Test plan

• pnpm lefthook validate
• PR_TITLE="ci: add lefthook workflow" pnpm lefthook run pre-commit --force
• node --experimental-transform-types tools/verify-pr-title.ts --title "fix(codescythe_cli)!: handle config lookup"
• git diff --check
• PR checks: Verify hooks, Test Rust