feat(proto): port caro protobuf schemas with com.miti99.gomoku.proto namespace

Copies request.proto and response.proto from caro, renames package to
com.miti99.gomoku.proto, drops Java options, adds go_package option,
and appends SpectatorCannotActResponse (field 21) to the Response oneof.

common/proto/request.proto

@@ -0,0 +1,43 @@
+syntax = "proto3";
+
+package com.miti99.gomoku.proto;
+
+option go_package = "github.com/tiennm99/gomoku/server/protocol;protocol";
+
+// Wrapper for all client -> server messages. The oneof case IS the event code.
+message Request {
+  oneof payload {
+    HeartbeatRequest heartbeat = 1;
+    SetNicknameRequest set_nickname = 2;
+    SetClientInfoRequest set_client_info = 3;
+    CreateRoomRequest create_room = 4;
+    CreatePveRoomRequest create_pve_room = 5;
+    GetRoomsRequest get_rooms = 6;
+    JoinRoomRequest join_room = 7;
+    GameStartingRequest game_starting = 8;
+    GameReadyRequest game_ready = 9;
+    GameMoveRequest game_move = 10;
+    GameResetRequest game_reset = 11;
+    WatchGameRequest watch_game = 12;
+    WatchGameExitRequest watch_game_exit = 13;
+    ClientExitRequest client_exit = 14;
+  }
+}
+
+message HeartbeatRequest {}
+message SetNicknameRequest { string nickname = 1; }
+message SetClientInfoRequest { string version = 1; }
+message CreateRoomRequest {}
+message CreatePveRoomRequest { int32 difficulty = 1; }
+message GetRoomsRequest {}
+message JoinRoomRequest { int32 room_id = 1; }
+message GameStartingRequest {}
+message GameReadyRequest {}
+message GameMoveRequest {
+  int32 row = 1;
+  int32 col = 2;
+}
+message GameResetRequest {}
+message WatchGameRequest { int32 room_id = 1; }
+message WatchGameExitRequest {}
+message ClientExitRequest {}

common/proto/response.proto

@@ -0,0 +1,121 @@
+syntax = "proto3";
+
+package com.miti99.gomoku.proto;
+
+option go_package = "github.com/tiennm99/gomoku/server/protocol;protocol";
+
+// Wrapper for all server -> client messages. The oneof case IS the event code.
+message Response {
+  oneof payload {
+    ClientConnectResponse client_connect = 1;
+    NicknameSetResponse nickname_set = 2;
+    ShowOptionsResponse show_options = 3;
+    ShowRoomsResponse show_rooms = 4;
+    RoomCreateSuccessResponse room_create_success = 5;
+    RoomJoinSuccessResponse room_join_success = 6;
+    RoomJoinFailFullResponse room_join_fail_full = 7;
+    RoomJoinFailNotFoundResponse room_join_fail_not_found = 8;
+    RoomPlayFailNotFoundResponse room_play_fail_not_found = 9;
+    GameStartingResponse game_starting = 10;
+    GameReadyResponse game_ready = 11;
+    GameMoveSuccessResponse game_move_success = 12;
+    GameMoveInvalidResponse game_move_invalid = 13;
+    GameMoveOccupiedResponse game_move_occupied = 14;
+    GameMoveOutOfBoundsResponse game_move_out_of_bounds = 15;
+    GameMoveNotYourTurnResponse game_move_not_your_turn = 16;
+    GameOverResponse game_over = 17;
+    PveDifficultyNotSupportResponse pve_difficulty_not_support = 18;
+    WatchGameSuccessResponse watch_game_success = 19;
+    ClientExitResponse client_exit = 20;
+    SpectatorCannotActResponse spectator_cannot_act = 21;
+  }
+}
+
+message ClientConnectResponse { int32 client_id = 1; }
+
+// invalid_length = 0 means "prompt only — ask the user for a nickname"
+message NicknameSetResponse { int32 invalid_length = 1; }
+
+message ShowOptionsResponse {}
+
+message RoomSummary {
+  int32 room_id = 1;
+  string room_owner = 2;
+  int32 room_client_count = 3;
+  string room_type = 4;
+}
+
+message ShowRoomsResponse { repeated RoomSummary rooms = 1; }
+
+message RoomCreateSuccessResponse {
+  int32 id = 1;
+  string room_owner = 2;
+  string room_type = 3;
+}
+
+message RoomJoinSuccessResponse {
+  int32 client_id = 1;
+  string client_nickname = 2;
+  int32 room_id = 3;
+  string room_owner = 4;
+  int32 room_client_count = 5;
+}
+
+message RoomJoinFailFullResponse {
+  int32 room_id = 1;
+  string room_owner = 2;
+}
+
+message RoomJoinFailNotFoundResponse { int32 room_id = 1; }
+
+message RoomPlayFailNotFoundResponse {}
+
+message GameStartingResponse {
+  int32 room_id = 1;
+  int32 black_player_id = 2;
+  string black_player_nickname = 3;
+  int32 white_player_id = 4;
+  string white_player_nickname = 5;
+  int32 board_size = 6;
+}
+
+message GameReadyResponse {
+  string client_nickname = 1;
+  string status = 2;
+  int32 client_id = 3;
+}
+
+message GameMoveSuccessResponse {
+  int32 row = 1;
+  int32 col = 2;
+  string piece = 3;
+  string player_nickname = 4;
+  int32 player_id = 5;
+}
+
+message GameMoveInvalidResponse {}
+message GameMoveOccupiedResponse {}
+message GameMoveOutOfBoundsResponse {}
+message GameMoveNotYourTurnResponse {}
+
+message GameOverResponse {
+  string result = 1;
+  string winner_nickname = 2;
+}
+
+message PveDifficultyNotSupportResponse {}
+
+message WatchGameSuccessResponse {
+  string owner = 1;
+  string status = 2;
+}
+
+message ClientExitResponse {
+  int32 room_id = 1;
+  int32 exit_client_id = 2;
+  string exit_client_nickname = 3;
+}
+
+// Sent when a spectator attempts an action that requires being a player.
+// Phase-07 dispatch sends this in response to GameMoveRequest from a spectator.
+message SpectatorCannotActResponse {}