[refactor] adopt yamux-only tunnel transport [BREAK]

- replace gRPC/DTLS tunnel paths with TLS + yamux transport
- remove legacy protocol, protobuf, DTLS, and proxy implementations
- simplify server and client entrypoints around yamux
- update configuration, metrics, API, and architecture documentation
- add bidirectional yamux session tests
- verify with go test ./...

.env.example

@@ -41,8 +41,8 @@ HOP_SERVER_HTTP_LISTEN=:8080
 # HTTPS 리스닝 포트 (보통 :443)
 HOP_SERVER_HTTPS_LISTEN=:8443
 
-# DTLS 리스닝 포트 (보통 :443, 필요시 별도 포트 사용)
-HOP_SERVER_DTLS_LISTEN=:8443
+# TLS + yamux 클라이언트 터널 포트
+HOP_SERVER_TUNNEL_LISTEN=:7443
 
 # 메인 도메인 (예: example.com)
 HOP_SERVER_DOMAIN=example.com
@@ -102,9 +102,9 @@ HOP_DB_DSN=postgres://user:pass@localhost:5432/hopgate?sslmode=disable
 
 # ---- Client settings ----
 
-# DTLS 서버 주소 (host:port)
-# 예: example.com:443
-HOP_CLIENT_SERVER_ADDR=localhost:8443
+# yamux 터널 서버 주소 (host:port)
+# 예: example.com:7443
+HOP_CLIENT_SERVER_ADDR=localhost:7443
 
 # 클라이언트 도메인
 HOP_CLIENT_DOMAIN=test.example.com