Let Codex discover the marketplace from the repository root

Constraint: Codex Git sparse checkout preserves selected paths, so plugins/codex is not treated as the marketplace root by the CLI.\nRejected: Require users to rely on a single plugins/codex sparse path | The CLI leaves the manifest nested and rejects the marketplace root.\nConfidence: high\nScope-risk: narrow\nDirective: Keep the root .agents/plugins/marketplace.json as the primary Git entrypoint.\nTested: python json.tool on both marketplace manifests; validate_plugin.py.\nNot-tested: Remote Git marketplace add after this commit.

.agents/plugins/marketplace.json

@@ -0,0 +1,20 @@
+{
+  "name": "eapil-skill-market",
+  "interface": {
+    "displayName": "EAPIL Skill Market"
+  },
+  "plugins": [
+    {
+      "name": "eapil-skills",
+      "source": {
+        "source": "local",
+        "path": "./plugins/codex/plugins/eapil-skills"
+      },
+      "policy": {
+        "installation": "AVAILABLE",
+        "authentication": "ON_INSTALL"
+      },
+      "category": "Developer Tools"
+    }
+  ]
+}