Add SKF external parser groundwork

[PRAteek-singHWY]

Summary

This PR adds a new backend parser for OWASP Security Knowledge Framework (SKF).
It scans SKF knowledge base markdown, code examples, and lab items, and only creates OpenCRE entries when those sources include OpenCRE links. No UI changes.

fixes #145

What We Did

• Added a new SKF parser that scans:
  • skf/markdown/knowledge_base/**.md
  • skf/markdown/code_examples/**.md
  • skf/initial_data.py (LabItem entries)
• Creates Standard/Tool nodes only if OpenCRE links exist in those files.
• Added CLI flag --skf_in and wired SKF into the external import registry.

What’s Done

• Parser and CLI wiring are complete.
• The pipeline is ready to import SKF data once OpenCRE links exist upstream.

Out of Scope

• Adding OpenCRE links to the SKF repo itself (needs an upstream PR).
• Any frontend/UI changes.

Why This Approach

This follows the maintainable workflow suggested in issue #145:
add OpenCRE links in the source documents, then let the parser import automatically.

[PRAteek-singHWY]

Hi @northdpole @robvanderveer
I followed the guidance from issue #145. This PR adds an SKF parser that scans SKF knowledge base markdown, code example markdown, and LabItem entries, and only creates OpenCRE entries when OpenCRE links are present in those sources. It also adds a --skf_in CLI flag and wires SKF into the external importer registry.

This keeps the integration maintainable and reciprocal, per Rob’s suggestion.
Remaining work is upstream: SKF needs to add OpenCRE links in their KB/code examples/labs for the importer to actually create mappings.

No frontend/UI changes.