Macro refactor ideas

1. Macro ergonomics + errors

• Emit compile-time errors when a macro is attached to the wrong target or missing descriptor/init.
• Add a member macro to generate static let descriptor from makeDescriptor() to remove boilerplate.

2. Descriptor/data shape

• Split ProviderDescriptor into Descriptor + FetchPlan + Branding + CLI files for cleaner deps.
• Move source label into fetch results (strategy-specific).

3. Fetching pipeline

• Return all attempted strategies + errors for debug UI and CLI --verbose.

4. Registry order & stability

• Use ProviderDescriptorRegistry registration order for all; no sorting by rawValue.
• Use metadata flags (e.g. isPrimaryProvider) instead of hard-coded Codex/Claude.

5. Account/identity

• Replace Codex UI special casing with a descriptor flag (e.g. usesAccountFallback).
• Enforce provider-keyed identity in snapshots to avoid cross-provider display.

6. Settings + tokens

• Move Zai/Copilot token lookup into strategy-local resolvers (Keychain/env).
• Add optional per-provider settings fields in ProviderSettingsSnapshot.

7. Docs + tests

• Add a minimal provider example in docs/provider.md.
• Add registry completeness + deterministic-order test.