feat(catalog): load models/providers from ~/.openfang catalog files

[lvy010]

Summary

• Load model/provider catalog from ~/.openfang/catalog instead of runtime hardcoded defaults in kernel/CLI paths.
• Introduce home-backed catalog loading to make model/provider visibility file-driven.
• Keep auth detection and provider URL override behavior intact.

Test

• [√] rm -rf ~/.openfang && ./target/debug/openfang init --quick
• [√] Verify ~/.openfang/catalog/{models,providers,aliases}.json exists
• [√] Edit providers.json and confirm openfang models providers --json reflects changes immediately
• [√] Set empty catalog files and verify models list/providers do not crash