fix: update SDK protocol version to v2

Copilot CLI v0.0.394 uses protocol version 2.
Update kSdkProtocolVersion from 1 to 2 to match.

Author: 0xeb

include/copilot/types.hpp

@@ -32,7 +32,7 @@ struct SessionEvent;
 // =============================================================================
 
 /// SDK protocol version - must match copilot-agent-runtime server
-inline constexpr int kSdkProtocolVersion = 1;
+inline constexpr int kSdkProtocolVersion = 2;
 
 // =============================================================================
 // Enums

tests/test_types.cpp

@@ -158,12 +158,12 @@ TEST(TypesTest, PingResponse)
     auto resp = input.get<PingResponse>();
     EXPECT_EQ(resp.message, "pong");
     EXPECT_EQ(resp.timestamp, 1234567890);
-    EXPECT_EQ(resp.protocol_version, 1);
+    EXPECT_EQ(resp.protocol_version, 2);
 }
 
 TEST(TypesTest, ProtocolVersion)
 {
-    EXPECT_EQ(kSdkProtocolVersion, 1);
+    EXPECT_EQ(kSdkProtocolVersion, 2);
 }
 
 TEST(TypesTest, SessionMetadataParsesIso8601Timestamps)