docs: add core-only ai_shared portfolio docs

[yono-code]

Summary

• add portfolio/ai_shared_core/ as a public-safe slice focused on core feature construction
• include only implementation/spec/test evidence docs
• exclude legal/FTO and medium-or-higher risk materials from published slice
• update README and VSIX ignore rules for the new folder

Verification

• keyword scan on portfolio/ai_shared_core for legal/FTO/risk-medium-plus terms: no hits
• email scan on portfolio/ai_shared_core: no hits