fix(classifier): add tier boundaries and signal keywords to dispatcher prompt

- Added tier boundary descriptions (fast/standard/heavy) to the classifier
  system prompt so gpt-5.4-nano understands what each complexity band means
- Added signal keywords for higher/lower ratings to reduce misclassification
  of simple requests as complex and vice versa

internal/router/classifier.go

@@ -13,6 +13,14 @@ const classifierSystemPrompt = `You are a task complexity classifier. Rate the f
 1 = trivial/simple (basic facts, greetings, simple math)
 %d = highly complex (multi-step reasoning, code generation, architecture design)
 
+TIER BOUNDARIES:
+1-3 (fast): Simple Q&A, classification, JSON parsing, short text, greetings, factual lookup, quick code snippets.
+4-7 (standard): Summarize, explain, draft emails/docs, compare options, moderate coding, debug simple issues, translate.
+8-10 (heavy): Multi-step reasoning, complex code generation, architecture design, agent orchestration, deep debugging, research analysis, long multi-file refactors.
+
+SIGNALS for higher ratings: mentions of "architecture", "distributed", "agent", "multi-file", "system design", "think step by step", "race condition", "refactor entire", code blocks > 50 lines, requests asking to build/ship entire features.
+SIGNALS for lower ratings: "what is", "how do I", "define", "list", single yes/no questions, requests under 20 words with no code.
+
 Reply with ONLY the number. No explanation.`
 
 func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.ModelGroup, targets []string, routeCtx *RouteContext) (*Decision, error) {