feat: implement Tier 3 features — lane queue, credential redaction, token dashboard, xAI, Voyage AI

- Lane Queue: per-session FIFO queue in gateway replacing reject-when-busy (9 tests)
- Credential Redaction: redactConfig() expanded to cover 18+ secret fields (16 tests)
- Web UI Token Dashboard: system.tokenUsage endpoint + Usage page with summary cards
- xAI (Grok) Provider: OpenAI-compatible client with model pricing
- Voyage AI Embeddings: new embedding provider with configurable dimensions (5 tests)
- Update gap analysis: 90→95 match (70%→74%), Tier 3 section marked DONE
- Update state.json: test count 1001→1034, add tier3_completion entry

Total: 1034 tests passing across 85 files, typecheck clean

src/frontends/tui/minimal.ts

@@ -381,7 +381,7 @@ export class MinimalTui {
             fullContent += event.content;
           }
           if (event.type === 'fallback_warning' && event.fallbackReason) {
-            console.warn(`\n⚠ ${event.fallbackReason}`);
+            console.warn(`\n⚠ Using fallback model`);
           }
           if (event.type === 'done' && event.usage) {
             this.totalUsage.inputTokens += event.usage.inputTokens;