Add Microsoft Careers site fetcher

[celloopa]

Summary

Closes #18

• Added specialized extractor for careers.microsoft.com and apply.careers.microsoft.com URLs
• Parses job data from Next.js __NEXT_DATA__ JSON embedded in page
• Extracts title, description, qualifications, responsibilities, location, and employment type
• Falls back to meta tags when JSON parsing fails
• Validates data to reject numeric company names and empty positions

Changes

• internal/fetch/fetcher.go - Added extractMicrosoft() and helper functions:
  • parseMicrosoftNextData() - Navigates JSON structure
  • extractMicrosoftJobData() - Extracts job fields
  • validateMicrosoftExtraction() - Data validation
  • isNumeric() - Helper for validation
• internal/fetch/fetcher_test.go - Added 7 new tests
• README.md - Added Microsoft Careers to supported job boards
• CHANGELOG.md - Added feature to Unreleased section
• PROGRESS.md - Updated task status and added completion log

Test plan

• All existing tests pass (go test ./...)
• New tests cover:
  • Full __NEXT_DATA__ JSON parsing
  • Meta tag fallback when no JSON
  • Alternative JSON structures (jobDetail vs job)
  • Array handling for qualifications/responsibilities
  • Data validation (numeric rejection)
  • apply.careers.microsoft.com subdomain support

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