[Beginner]: Extract User-Agent string in `fetch_workday_jobs` into a global constant

Description

There is a hardcoded Mozilla/5.0... User-Agent string inside fetch_workday_jobs(). Extract this into a global constant DEFAULT_USER_AGENT.

Why it matters

Duplicating or hiding user agent strings makes them hard to update universally if the scraper gets blocked.

Acceptance Criteria

• [ ] Create GLOBAL_USER_AGENT constant
• [ ] Use it in the Workday headers dictionary
• [ ] Passes pre-commit run --all-files