[codex] Correct spending policy enforcement copy

[Dreamstore2046]

Summary

• replace the Policy tab's Soroban smart-contract claim with the requested server-side enforcement copy
• expand docs/SPENDING-POLICY.md with the real enforcement model, threat model, and enforcement limits
• add a PolicyTab inline snapshot regression test for the updated text and assert the Soroban claim is gone

Closes #25

Validation

• npm test -- dashboard/src/__tests__/policy-tab.test.tsx
• git diff --cached --check

Notes

• A focused TSX typecheck is still blocked in this checkout by existing dashboard typing issues, including missing root-resolvable React declarations and pre-existing PolicyTab type mismatches unrelated to this copy change.

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