docs: record chat secure-fact rejection proof

[thesongzhu]

Summary

• record the focused real-browser /chat generated secure_fact rejection proof in docs/current-source-of-truth.md
• keep broader generated-candidate rejection, third-party channels, attachments/documents, every sensitive category, cross-device, and full secure-storage UX proof-pending

Local checks

• git diff --check
• npm run check:secret-patterns
• npm run check:alignment
• npm run check:public-source-hygiene