fix(start-work): restore atlas agent and proper model fallback chain

- Restore agent: 'atlas' in start-work command (removed by PR #1201)
- Fix model-resolver to properly iterate through fallback chain providers
- Remove broken parent model inheritance that bypassed fallback logic
- Add model-suggestion-retry for runtime API failures (cherry-pick 800846c1)

Fixes #1200

src/shared/index.ts

@@ -32,3 +32,4 @@ export * from "./connected-providers-cache"
 export * from "./case-insensitive"
 export * from "./session-utils"
 export * from "./tmux"
+export * from "./model-suggestion-retry"