Add id slug field to metadata

[marcarl]

Summary

• Adds a new id field at the top of the YAML frontmatter in all documents
• The id is generated from beteckningSortable in the format sfs-YYYY-NNN (e.g., sfs-2024-1000)
• Refactors filename generation to use the id slug (filenames remain unchanged: sfs-2024-1000.md)
• Updates frontmatter sorting to ensure id appears first

Changes

• sfs_processor.py:
  • New create_id_slug() function to generate clean slug from beteckningSortable
  • Updated create_safe_filename() to use id slug as input (filenames unchanged)
  • Modified frontmatter generation to include id field at the top
  • The id field value matches the filename (without .md extension)
• formatters/sort_frontmatter.py:
  • Added id to PROPERTY_ORDER as the first field

Example

---
id: sfs-2010-800
beteckning: "2010:800"
rubrik: Skollag
normtyp: Lag
...
---

Filename: sfs-2010-800.md

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