feat(agentcompat): add shared runtime contracts

Co-authored-by: naiba/CloudCode <hi+cloudcode@nai.ba>

pkg/agentcompatcontract/header.go

@@ -0,0 +1,3 @@
+package agentcompatcontract
+
+const IOStreamCapabilityHeader = "X-Nezha-AgentCompat-IOStream-Capability"

pkg/agentcompatcontract/header_test.go

@@ -0,0 +1,9 @@
+package agentcompatcontract
+
+import "testing"
+
+func TestIOStreamCapabilityHeaderUsesFrozenName(t *testing.T) {
+	if IOStreamCapabilityHeader != "X-Nezha-AgentCompat-IOStream-Capability" {
+		t.Fatalf("unexpected capability header name")
+	}
+}