ci: add PR title lint workflow (#461)

## Summary
Adds PR title validation modeled after langchain-ai/langchainjs#10703.

## Changes
- Harden .github/workflows/pr_lint.yml
- Reject empty scopes in PR titles (for example, fix(): ...)
- Enforce Conventional Commit title format with semantic PR checks
- Add repo-relevant scopes and disallow release/uppercase scopes
- Keep ignore-lint-pr-title label override