fix(model-fallback): respect user model override for sisyphus-junior category sessions (#2941)

When users configure agents.sisyphus-junior.model, category sessions
should use that model without falling back to hardcoded
AGENT_MODEL_REQUIREMENTS. Two leaks existed:

1. Launch-time: category-resolver.ts did not check overrideModel when
   deciding whether to use the hardcoded fallbackChain. Fixed by adding
   overrideModel to the suppression condition.

2. Runtime retry: model-fallback hook.ts treated undefined fallbackChain
   (from setSessionFallbackChain(id, undefined)) as 'no override' and
   fell through to AGENT_MODEL_REQUIREMENTS on session.error/status
   retry. Fixed by storing explicit empty array and checking .has()
   instead of truthiness.

Tests added:
- hook.test.ts: explicit empty session fallback suppresses agent chain
- event.model-fallback-2941.test.ts: session.error does not arm retry
- category-resolver.test.ts: overrideModel suppresses fallbackChain
- tools.test.ts: sisyphusJuniorModel override assertion

src/tools/delegate-task/category-resolver.ts

@@ -275,6 +275,6 @@ Available categories: ${categoryNames.join(", ")}`,
     actualModel,
     isUnstableAgent,
     // Don't use hardcoded fallback chain when resolution was skipped (cold cache)
-    fallbackChain: configuredFallbackChain ?? ((isModelResolutionSkipped || explicitCategoryModel) ? undefined : requirement?.fallbackChain),
+    fallbackChain: configuredFallbackChain ?? ((isModelResolutionSkipped || explicitCategoryModel || overrideModel) ? undefined : requirement?.fallbackChain),
   }
 }