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.test.ts

@@ -484,4 +484,32 @@ describe("resolveCategoryExecution", () => {
 		})
 		expect(result.fallbackChain).toBeUndefined()
 	})
+
+	test("does not inherit hardcoded fallbackChain when sisyphus-junior model override is set [regression #2941]", async () => {
+		//#given
+		const args = {
+			category: "quick",
+			prompt: "test prompt",
+			description: "Test task",
+			run_in_background: false,
+			load_skills: [],
+			blockedBy: undefined,
+			enableSkillTools: false,
+		}
+		const executorCtx = createMockExecutorContext()
+		executorCtx.sisyphusJuniorModel = "anthropic/claude-sonnet-4-6"
+
+		//#when
+		const result = await resolveCategoryExecution(args, executorCtx, undefined, "anthropic/claude-sonnet-4-6")
+
+		//#then
+		expect(result.error).toBeUndefined()
+		expect(result.actualModel).toBe("anthropic/claude-sonnet-4-6")
+		expect(result.categoryModel).toEqual({
+			providerID: "anthropic",
+			modelID: "claude-sonnet-4-6",
+			variant: undefined,
+		})
+		expect(result.fallbackChain).toBeUndefined()
+	})
 })

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),
   }
 }

src/tools/delegate-task/tools.test.ts

@@ -2963,6 +2963,7 @@ describe("sisyphus-task", () => {
       // then - sisyphus-junior override model should be used, not category default
       expect(launchInput.model.providerID).toBe("anthropic")
       expect(launchInput.model.modelID).toBe("claude-sonnet-4-6")
+      expect(launchInput.fallbackChain).toBeUndefined()
     })
 
     test("sisyphus-junior model override works with user-defined category (#1295)", async () => {