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/hooks/model-fallback/hook.test.ts

@@ -332,6 +332,25 @@ describe("model fallback hook", () => {
     clearPendingModelFallback(sessionID)
   })
 
+  test("does not fall back to hardcoded agent chain when session explicitly stores no fallback chain [regression #2941]", () => {
+    //#given
+    const sessionID = "ses_model_fallback_explicit_none"
+    clearPendingModelFallback(sessionID)
+    setSessionFallbackChain(sessionID, undefined)
+
+    //#when
+    const set = setPendingModelFallback(
+      sessionID,
+      "Sisyphus - Junior",
+      "anthropic",
+      "claude-sonnet-4-6",
+    )
+
+    //#then
+    expect(set).toBe(false)
+    clearPendingModelFallback(sessionID)
+  })
+
   test("shows toast when fallback is applied", async () => {
     //#given
     const toastCalls: Array<{ title: string; message: string }> = []