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/features/builtin-commands/commands.ts

@@ -55,6 +55,7 @@ ${REFACTOR_TEMPLATE}
   },
   "start-work": {
     description: "(builtin) Start Sisyphus work session from Prometheus plan",
+    agent: "atlas",
     template: `<command-instruction>
 ${START_WORK_TEMPLATE}
 </command-instruction>