feat(showcase): add eval tier configuration and gitignore entries

Add eval-tiers.json defining three evaluation tiers (Gold Standard,
Key Partners, Full Matrix) with fail-fast on tier 1. Add gitignore
entries for .eval-results/ and .eval-baseline.json local artifacts.

Author: jpr5

showcase/.gitignore

@@ -19,3 +19,7 @@ shell/src/data/*.json
 shell-dojo/src/data/*.json
 shell-docs/src/data/*.json
 shell-dashboard/src/data/*.json
+
+# Eval results (local, not committed)
+.eval-results/
+.eval-baseline.json

showcase/eval-tiers.json

@@ -0,0 +1,20 @@
+{
+  "_comment": "Tiered evaluation order. Tier 1 fail_fast stops eval early on gold-standard regression.",
+  "tiers": [
+    {
+      "name": "Gold Standard",
+      "slugs": ["langgraph-python"],
+      "fail_fast": true
+    },
+    {
+      "name": "Key Partners",
+      "slugs": ["mastra", "crewai-crews", "google-adk", "langgraph-typescript"],
+      "fail_fast": false
+    },
+    {
+      "name": "Full Matrix",
+      "slugs": "*",
+      "fail_fast": false
+    }
+  ]
+}