feat: conformance scenarios (core-01..core-09)

Declarative conformance scenarios for aicc/0.1 covering: handshake,
manifest_request, tool call ok/unknown/execution-failed, protocol
mismatch, heartbeat, session close, event delivery. Plus runner
documentation (conformance/README.md).

conformance/scenarios/core-02-manifest-request.json

@@ -0,0 +1,20 @@
+{
+  "id": "core-02-manifest-request",
+  "title": "manifest_request returns the same session_init",
+  "category": "core",
+  "version": "aicc/0.1",
+  "description": "A manifest_request must be answered with the session manifest for the current session.",
+  "steps": [
+    {
+      "send": {
+        "type": "manifest_request"
+      }
+    },
+    {
+      "expect": {
+        "type": "session_init",
+        "session_id_echo": true
+      }
+    }
+  ]
+}