Support custom skill taxonomies via YAML override

[akuligowski9]

Summary

Allow users to extend or override the built-in skill taxonomy with a custom YAML file, enabling ProjectBridge to work for niche domains (embedded systems, bioinformatics, game dev, etc.) that aren't covered by the default 71 skills.

Details

The current taxonomy in engine/projectbridge/analysis/taxonomy.py is a hardcoded dict of ~71 skills with adjacency relationships. Users in specialized fields can't get useful results because their core technologies aren't recognized.

Proposed approach

1. Define a YAML schema for custom taxonomies (skill name, category, adjacent skills)
2. Load from a conventional path (e.g., ~/.config/projectbridge/taxonomy.yaml or a --taxonomy CLI flag)
3. Merge custom entries into the built-in taxonomy — custom entries override built-in ones with the same key, new entries are added
4. Validate adjacency references (warn if a custom skill references a non-existent adjacent skill)

References

• Built-in taxonomy: engine/projectbridge/analysis/taxonomy.py
• Config loading pattern: engine/projectbridge/config/settings.py
• Analysis engine that consumes the taxonomy: engine/projectbridge/analysis/engine.py