style: check error returns flagged by errcheck linter

golangci-lint's errcheck rule requires explicit handling of error
return values from Close, Write, and Logout calls. Use blank
identifier assignment for cleanup paths where errors are
intentionally discarded.

cmd/protbot/protocol/entrance.go

@@ -24,7 +24,7 @@ func DoEntrance(addr string) ([]ServerEntry, error) {
 	if err != nil {
 		return nil, fmt.Errorf("entrance connect: %w", err)
 	}
-	defer c.Close()
+	defer func() { _ = c.Close() }()
 
 	// Send a minimal packet (the entrance server reads it, checks len > 5 for USR data).
 	// An empty/short packet triggers only SV2 response.

cmd/protbot/protocol/sign.go

@@ -25,7 +25,7 @@ func DoSign(addr, username, password string) (*SignResult, error) {
 	if err != nil {
 		return nil, fmt.Errorf("sign connect: %w", err)
 	}
-	defer c.Close()
+	defer func() { _ = c.Close() }()
 
 	// Build DSGN request: "DSGN:041" + \x00 + SJIS(user) + \x00 + SJIS(pass) + \x00 + \x00
 	// The server reads: null-terminated request type, null-terminated user, null-terminated pass, null-terminated unk.