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/background-agent/manager.ts

@@ -5,7 +5,7 @@ import type {
   LaunchInput,
   ResumeInput,
 } from "./types"
-import { log, getAgentToolRestrictions } from "../../shared"
+import { log, getAgentToolRestrictions, promptWithModelSuggestionRetry } from "../../shared"
 import { ConcurrencyManager } from "./concurrency"
 import type { BackgroundTaskConfig, TmuxConfig } from "../../config/schema"
 import { isInsideTmux } from "../../shared/tmux"
@@ -307,7 +307,7 @@ export class BackgroundManager {
       : undefined
     const launchVariant = input.model?.variant
 
-    this.client.session.prompt({
+    promptWithModelSuggestionRetry(this.client, {
       path: { id: sessionID },
       body: {
         agent: input.agent,

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>