Consolidate financial intelligence design docs

[codeforester]

Summary

• Replace the architecture review/source-spec pair with one canonical financial intelligence architecture doc.
• Keep the vision doc as the short product north star and move unresolved topics into a living open-questions doc.
• Delete stale AssetBuddy/source-spec docs and update README plus AI context references.

Validation

• ./tests/validate.sh
• git diff --check
• git diff --cached --check

Closes #123