feat(ats): implement LinkedIn Skills API integration

[athola]

Context

The SkillsTaxonomyFetcher._fetch_linkedin() method in src/simple_resume/core/ats/taxonomy.py is currently a stub.

Requirements

• Implement LinkedIn Skills API integration
• Requires OAuth authentication setup
• Should work with the existing TaxonomyCache protocol for local caching
• Graceful fallback to HARDCODED_SKILLS on failure

References

• Related to feat(ats): add optional skills taxonomy API integration #59 (offline-first skills taxonomy API integration)
• PR feat: implement issues #48 #50 #78 #79 and ATS enhancements #80 review comment