Keep News agent overrides app-owned

[hamzamerzic]

Summary

• translate News's current modes and legacy bare-provider settings into the platform's uniform agent-override contract
• preserve inherited system choices and app-owned primary/fallback choices
• keep provider, model, and effort normalization in the shared platform resolver

Why

Follow-up to mobius-os/mobius#516 and #4. The platform no longer interprets each app's settings format, so News must translate its own current and legacy shapes before calling the shared resolver. Without that adapter, app-specific agent choices are silently treated as absent.

Verification

• npm test (103 tests)
• bash -n fetch.sh
• package validation (0 errors; the existing Pocket TTS cross-origin warning is unchanged)
• git diff --check