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/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 () => {